The trick is to then talk through the code with them in the next interview. "Why did you do this part this way? Would you do it differently if requirement X we

The trick is to then talk through the code with them in the next interview. "Why did you do this part this way? Would you do it differently if requirement X were different?" It's immediately obvious if they cheated. Better yet, that's a much better, more relevant discussion than most whiteboard problems.