One of the best interviews I've had as a candidate (didn't get that job - it's the one that got away) involved me explaining design choices I made while implementing my solution to the take-home test. That was a great discussion, and didn't feel like the traditional interview-style questioning.
We ask candidates to submit code samples of their own choosing, or if they have nothing that they can supply for whatever reason we give them a short challenge which is only used for generating code to review.
We bring them onsite and review the code and have them walk through what's going on. If they just blatantly copied stuff or handed in someone else's code they won't be able to stand up to scrutiny. And if they really do know it inside and out they probably could have written it anyways. We've had a small number of candidates who tried to pull a fast one but very very few
