The best I have come up with if you are requiring them to write actual code is to give them a problem to solve for with code prior to interviewing. Have them br

The best I have come up with if you are requiring them to write actual code is to give them a problem to solve for with code prior to interviewing. Have them bring that code along and then have them explain the code and why they did things the way they did. It is not 100% verifiable that they wrote the code, but being able to talk to why things were done the way they were tends to be good enough. I have also found that understanding how people approach life and problems in general are more telling of how good they will be than actually evaluating their coding skills. Monitoring coding exercises will weed out many good candidates as well as the poor ones in my experience.