If an omnipotent being wants you to believe something that isn't true, and is willing to use its omnipotence to convince you of the truth of that untruth, then there is nothing you can do about it. There is no observation that suffices to prove that an omnipotent being is telling the truth, as a malevolent omnipotence could make you believe literally anything - that you observed any given observation - or didn't, or that impossible things make sense, or sensible things are impossible.

This is one of a larger class of questions where one answer means that you are unable to assume the truth of your own knowledge. There is nothing you can do with any of them except smile at your own limitedness, and make the assumption that the self-defeating answer is wrong.

There is some confusion here. Asking Less Wrong flavored questions using theological terms generally requires misusing the terms. This is unfortunate, because these questions are really interesting, but most us don't have the requisite understanding of theology to do it well (including myself obviously(although, I venture that I might know more than most(#nesting))). So, my answer will be really disappointing.

In the monotheistic theology of Islam (represented by Al-Farabi), Judaism (represented by Maimonides), and, Christianity (represented by Thomas Aquinas), when it is said that God is omnipotent, they are saying God is not lacking in power, not that God can actually will to do any particular action whatsoever. In this way God is restrained. For example, God cannot create a rock so heavy that he cannot lift it because that is not a logical possibility. Or as a mathematician once said, "Nonsense is nonsense, even if you say it about God."

To your question about a loving God's possibility to deceive. This is a tough question because it is several in one. Can God deceive, can God's nature be learned about through observation of the created universe, and can God deceive about his nature? The first two questions are contested within each faith tradition, the third question (which I think is most relevant here) third is not disputed by the three philosophers. They all would say, "No."

I'm going to summarize a really long arguments the best I can: since God is a self-caused simple being (having no parts and lacking in no quality), his intellect (it's an operation) can only be directed toward Truth and his will (it's his other operation) can only be directed toward the Good (which is love).

This argument requires that we agree that Truth and Goodness have a primary level of existence, whereas falsity and evil exist contingently on the existence of truth and goodness. Since God has no parts, he cannot be oriented towards the composite essences of falsity and evil.

This is definitely an unsatisfying solution for most of us. The major problem for us approaching Theological Epistemology, as I see it, is that we have to start by explaining what metaphysics we are willing to accept and what we aren't.

Oh, they are far more tricky than that. At least the Edward Feser type Scholastics. They just define love as whatever god does.

Basically, the logic is that to love someone is not to wish that they get what they want to get, but to wish that they get what is actually good for them. What is actually good for them is what is in accordance with their natural goal, telos, and this does not even contradict common sense much, to the Scholastic the natural goal of horses is to run fast, eat grass, mate etc. and yes, indeed if we wanted to build a horse heaven it would be huge grassland for long runs. So so far it even checks out for the common-sense observer.

Then they just say natural goals are determined by god and QED.

Scholastics say a true god cannot be unloving by definition, in any logically conceivable universe, because it would imply he does not want what he wants (helping beings to reach natural goals he himself determined) In other words, they think the map / language logic is something superior to reality.

How does a theist distinguish by any imaginable experience between an omnipotent and loving Being, and an omnipotent Being that just wants you to believe it is loving?

How do you distinguish between a loving person and an actor who wants to appear loving? Well, you wait for the actor to trip up. If the actor is exceptionally good, you'll probably have to wait a long time. If the actor is infinitely good, you can't tell the difference.

If a being is able to perform a service to you that you are unable to recognise as being a service, there is no way to know whether a given act is a service or disservice.

Kids might think that their parents hate them for ruining their fun of running under cars.

That is while it might be somewhat easier to detect whether a omnipotent being regards you as you would regard someone you love it would be considerably harder to discern whether a limited powered being treats you as best of your interest as understood by them if you don't know how they understand you.