To my mind, the simplest and most straightforward way to combat any fraud is also beneficial because it gives you even more information about the candidate:
Talk to them about the code they wrote.
Have a conversation, as if they were already your co-worker, with the exercise as the subject. Go through it, ask them -- non-adversarially -- why they did X, what they thought about requirement 2, how they could get better test coverage for Z. If you see something that seems to be a mistake, talk about it. If you see something awesome, discuss it. I can't imagine someone incompetent being able to bullshit their way through detailed discussion of code they were supposed to have written.
And this provides valuable info about how this person thinks and communicates about the work you want them to do.
That seems like a good idea, but does somewhat detract from the notion that once a candidate does the work-sample he is done with the technical part of the process.
A nice approach I liked (back when I was interviewed by Thoughtworks years ago) was that one of the interview stages was to take the assignment you'd done and apply some new requirements to the story. It rapidly makes it clear if the candidate actually did the assignment or not.
Talk to them about the code they wrote.
Have a conversation, as if they were already your co-worker, with the exercise as the subject. Go through it, ask them -- non-adversarially -- why they did X, what they thought about requirement 2, how they could get better test coverage for Z. If you see something that seems to be a mistake, talk about it. If you see something awesome, discuss it. I can't imagine someone incompetent being able to bullshit their way through detailed discussion of code they were supposed to have written.
And this provides valuable info about how this person thinks and communicates about the work you want them to do.