Set a small piece of paid work. Stating that this could lead on to large pieces.
Obviously set some boundaries on this (time, language/ technology) then vet them on there performance.
Here is the thing.
There are many developers out there some good some not so good. What you need to do is find someone who is honest, hard working and is willing to learn.
My general rule is ask them what they have worked on in the past with links and give it a test it. Ask them a couple of easy questions to vet there personality more than code. Then ask them a question to which I hope they dont know the answer.
At this point I want them to say to me "I dont know, but I will find out" it shows to me they have the right attitude and know they are willing to say they dont know all the answers.
