Like anything else, the more you do it the easier it gets. I have two part time developers working for me (one from Nepal and the other from Ukraine. They're both great. Much better than the 80-90k/year developers I worked with when I had a day job.

Initially I start by posting the job, then filtering in stages before asking them (and paying them) to complete a small 1 to 2 hour coding project. The coding exercise can be done many different ways but the purpose is to evaluate problem solving, code quality, knowledge of libraries, code structure, etc. It's amazing how much insight you can gain with something like that.

More importantly, having them work on something real helps me further evaluate communication, responsiveness, and other attributes I look for in a good dev.