I don't have a great simple answer for this. I think it some combination of maybe looking at code samples, talking to previous employers, maybe doing some code

I don't have a great simple answer for this. I think it some combination of maybe looking at code samples, talking to previous employers, maybe doing some code in a timed exercise, either on or off-site, or maybe even contracting with a prospective employee to pay them to work on some small trial piece of code. I agree with you that there are definitely candidates out there who are all talk. I'm just not sure demanding code on a whiteboard during the interview itself is really a foolproof proxy for "smart and gets things done." I think it might weed out obvious duds but it might also weed out some people who might be having a bad day. It also is, I think, quite possible to game this kind of interview question, by studying some of the many books of coding interview questions coming out and just memorizing some of them.