Well, personality surveys can get away with using blatantly loaded questions because there's not much motivation to lie on an anonymous survey (though it's not exactly unheard of anyway, and there are issues of self-image vs. behavior to consider). This obviously wouldn't fly if something like college admissions or a job application was riding on the results, but I'm not going to condemn the existing tests for failing outside of the domain they were designed for.
I can't think off the top of my head of any good ways to measure conscientiousness in such an accurate and finely-grained way that the results could be used like IQ tests are used; you could use secret-test-of-character methods like e.g. leaving out a basket of donuts with a donation box and a hidden camera, but that'd give you only one data point. But I'm not going to say that designing a test like that is impossible, either.
you could use secret-test-of-character methods like e.g. leaving out a basket of donuts with a donation box and a hidden camera
That also has the problem that these things are easily gamed if their famous enough.
I've never heard of anyone saying "I thought that person was really intelligent, but they turned out not to be", and when there are scandals about people with fake credentials, they don't seem to come from people with fake credentials making mistakes-- instead, someone checks the history.
It seems to me that you can find out a lot about people's intelligence by talking with them a little, though I've underestimated people who were bright enough but didn't present as intellectual.
The real problems are with identifying conscientiousness, benevolence, and loyalty-- that's where the unpleasant surprises show up.