In my experience, the single best question for screening out overt charlatans is a simple "Tell me about a problem you've worked on recently." Everybody engaged with technical work has a few war stories, and they're hard to fabricate without a deep understanding of the domain (at which point they wouldn't be a charlatan, now would they). It generally relaxes nervous candidates a bit, since it's external enough to not feel like bragging, while being self-congratulatory in that candidates almost always talk about a problem they succeeded in solving (and the ones who don't are always talking about a very interesting problem). What kind of problem they describe tends to give you some insight into their strengths, and their general enthusiasm clues you into their interests. Throw in some probing questions if they don't go into enough detail, and you can come out the other end of it pretty confident whether they have technical chops or not.

So far, it's been extremely successful for me in predicting whether a candidates going to get weeded out by the rest of the interviewing process or not.