One way is to simulate a perfect computational agent, assume perfect information, and see what kind of models it would construct.

Right, but I meant, in practice.

that is, learning from observations.

Observations of what ? Since you do not have access to infinite computation or perfect observations in practice, you end up observing the outputs of models, as suggested in the original post.

For example, how about "relying on the accumulated knowledge of others"?

What is it that makes their accumulated knowledge worthy of being relied upon ?

We don't want to be confused with the uncritically thinking masses - the apologists of homoeopathy or astrology justifying their views by "yeah, I don't know how it works either, but it's useful!";

I think this statement underscores the problem with rejecting the correspondence theory of truth. Yes, one can say "homeopathy works", but what does that mean ? How do you evaluate whether any given model is useful of not ? If you reject the notion of an external reality that is accessible to us in at least some way, then you cannot really measure the performance of your models against any kind of a common standard. All you've got left are your internal thoughts and feelings, and, as it turns out, certain goals (such as "eradicate polio" or "talk to people very far away") cannot be achieved based on your feelings alone.

I've got feeling that the implicit LessWrong'ish rationalist theory of truth is, in fact, some kind of epistemic (Bayesian) pragmatism, i.e. "true is that what is knowable using probability theory". May also throw in "..for a perfect computational agent".

My speculation is that the declared LW's sympathy towards the correspondence theory of truth stems from political / social reasons. We don't want to be confused with the uncritically thinking masses - the apologists of homoeopathy or astrology justifying their views by "yeah, I don't know how it works either, but it's useful!"; the middle-school teachers who are ready to threat scientific results as epistemological equals of their favourite theories coming from folk-psychology, religious dogmas, or "common sense knowledge", because, you known, "they all are true in some sense". Pragmatic theories of truth are dangerous if they come into the wrong hands.

The correspondence theory of truth, however, describes truth as a correspondence relation between a model and the world itself. Not another model of the world, the world. And that, I contend, is impossible. We do not have direct access to the world.

What do you mean by "direct access to the world"? It seems natural to describe looking at something in the world as "access", reading of world's state, communicating information about its state to my mind. Just as information can be read from a hard drive and communicated to the state of a register in a CPU, similarly it can be read from the state of a tree outside my window and communicated to the state of my brain, or that of a camera taking a photo.

We may form a model of correspondence between the state of the tree and the state of a camera (between the tree and its photo). Such correspondence may result from camera's observation of the tree, the act of accessing the tree, that communicated information about the tree into the form of state of the camera. The model that describes the correspondence between the tree and the camera is not the same thing as the photo itself, perhaps a person may form that model.

The same situation occurs when instead of a photo in a camera there is knowledge in a mind. I may consider whether your knowledge of a tree corresponds to the tree. Or I may consider whether my own knowledge of a tree corresponds to the tree. The two models, the model of the tree and the model of the first model's correspondence to the tree, don't need to be represented by the same mind, but nothing changes when they are.

We may then further consider whether the second model (of the correspondence between the photo and the tree) corresponds to the actual correspondence between them, by considering how it formed in the mind etc. This third model may also be located in someone else's mind. For example, I may have looked at a photo and thought that it's accurate, but you may consider my judgement of photo's accuracy and decide that the judgement is wrong, that the photo does not represent what I thought it represented, that instead it represents something else.

There are just these strange artifacts of knowledge in people's heads that may be understood as relating all kinds of things in the world, including representations of knowledge, and the act of understanding them as relating things constitutes production of another artifact of that kind. Given that the parts of the world that hold these artifacts are in principle understood very well (building blocks like atoms etc.), pragmatically it doesn't matter whether "models are fundamental" or "reality is fundamental", in the sense that the structure of how representations of knowledge relate to things in the world (including other representations of knowledge) would be the same even if all mentions of reality are rewritten in a different language.

Science is the belief in the ignorance of experts.

Richard Feynman, What is Science?

Sounds like a form of abduction, or, more precisely, failure to consider alternative hypotheses.

As for your options, have you considered the possibility that 99% of people have never formulated a coherent philosophical view on the theory of truth?

I'd love to hear a more qualified academic philosopher discuss this, but I'll try. It's not that the other theories are intuitively appealing, it's that the correspondence theory of truth has a number of problems, such as the problem of induction.

Let's say the one day we create a complete simulation of a universe where the physics almost completely match ours, except some minor details, such as that some specific types of elementary particles, e.g. neutrinos are never allowed to appear. Suppose that there are scientists in the simulation, and they work out the Standard Model of their physics. The model presupposes existence of neutrinos, but their measurement devices are never going to interact with a neutrino. Is the statement "neutrinos exist" true or false from their point of view? I'd say that the answer is "does not matter". To turn the example around, can we be sure that aether does not exist? Under Bayesianism, every instance of scientists not observing aether increases our confidence. However we might be living in a simulation where the simulators have restricted all observations that could reveal the existence of aether. So it cannot be completely excluded that aether exists, but is unobservable. So the correspondence theory is forced to admit that "aether exists" has an unknown truth value. In contrast, a pragmatic theory of truth can simply say that anything that cannot, in principle, be observed by any means also does not exist, and be fine with that.

Ultimately, the correspondence theory presupposes a deep Platonism as it relies on the Platonic notion of Truth being "somewhere out there". It renders science vulnerable to problem of induction (which is not a real problem as far as real world is concerned) - it allows anyone to dismiss the scientific method off-handedly by saying that "yeah, but science cannot really arrive at the Truth - already David Hume proved so!"

We have somehow to deal with the possibility that everything we believe might turn out to be wrong (e.g. we are living in a simulation, and the real world has completely different laws of physics). Accepting correspondence theory means accepting that we are not capable of reaching truth, and that we are not even capable of knowing if we're moving in the direction of truth! (As our observations might give misleading results.) A kind of philosophical paralysis, which is solved by the pragmatic theory of truth.

There's also the problem that categories really do not exist in some strictly delineated sense; at least not in natural languages. For example consider the sentences in form "X is a horse". According to correspondence, a sentence from this set is true iff X is a horse. That seems to imply that X must be a mammal of genus Equus etc. - something with flesh and bones. However, one can point to a picture of a horse and say "this is a horse", and would not normally be considered lying. Wittgenstein's concept of family resemblance comes to rescue, but I suspect does not play nicely with the correspondence theory.

Finally, there's a problem with truth in formal systems. Some problems in some formal systems are known to be unsolvable; what is the truth value of statements that expand to such a problem? For example, consider the formula G (from Goedel's incompleteness theorem) expressed in Peano Arithmetic. Intuitively, G is true. Formally, it is possible to prove that assuming G is true does not lead to inconsistencies. To do that, we can provide a model of Peano Arithmetic using this standard interpretation. The standard set of integers is an example of such a model. However, it is also possible to construct nonstandard models of Peano Arithmetic extended with negation of G as an axiom. So assuming that negation of G is true also does not lead to contradictions. So we're back at the starting point - is G true? Goedel thought so, but he was a mathematical Platonist, and his views on this matter are largely discredited by now. Most do not believe that G has a truth value is some absolute sense.

This aspect together with Tarki's undefinability theorem suggest that is might not make sense to talk about unified mathematical Truth. Of course, formal systems are not the same as the real world, but the difficulty of formalizing truth in the former increases my suspicion of formalizations / axiomatic explanations relevant to in the latter.

Why would anyone bother to send in false data about their finger-length ratios?

The 2014 LW survey results mentioned something about being consistent with a finger-length/feminism connection. Maybe that counts?

Some diseases impact both reasoning and appearance. Gender impacts both appearance and behavior. You clearly get some information from appearance, but it's going to be noisy and less useful than what you'd get by just asking a few questions.