You could try to give them all a well-defined task and see how long it takes to deliver a working solution. Then mix it up, and send them a copy of someone els

You could try to give them all a well-defined task and see how long it takes to deliver a working solution. Then mix it up, and send them a copy of someone else's code and have each one extend it to do a second well-defined task. Repeat until you get a strong pattern which matches what you want to see in the real world. If you're lucky, this will show you who's capable of delivering working code, who's capable of turning in work in a reasonable amount of time, who writes maintainable code, and who can deal with maintaining code from other people. You might also rig things to see how they do at documentation, support, or other "meta" stuff, but that requires more rounds and more testing. Basically, create a fake programming team and throw some stuff at them, and then cull the ones who don't seem to be working out. If you can set it up so you make more money back than this costs you, then you win.