The problem is sometimes people believe lies they've been told -- they might have read an article somewhere that x can do y, when they really misinterpreted the

The problem is sometimes people believe lies they've been told -- they might have read an article somewhere that x can do y, when they really misinterpreted the entire thing, and hold that (mis)belief in their head. They might even sound confident when they talk as well, because to them it is true. That being said, if someone has knowledge in the expertise you are looking for, they should be able to break things down. So if you ask them a question about Z, and they say "well of course I can do Z", you ask them to break down what the steps they would go through in order to do Z. If they know their stuff, it should be trivial. On a side note, the people who know their limitations are, like you said, the ones you want to talk to. Socrates was considered the wisest man in Greece, it was said, because "[he] knew that [he] knew nothing"(paraphrased)