Easy objectives, difficult task. The first place I like to start is with their portfolio or resume. While skill sets are nice, I'm more concerned about accomplishments. If I see something which is 80% "My duties were....", Bam it goes into the trash. Results count. Show me what you did for whom. And why I should care.
The second criteria is even more important. Just because they may be the "best" in your field may still get you tossed. Do they play well with others? Here is where old written recommendation blurbs are great (love them on LinkedIn). Get to know them and talk to them. If unsure, start with a small project and see how it goes. I also look for people who are willing to call my baby ugly and then tell me how to make it better.
The final deciding point is the answer to this question:
It's 10:00 at night and the phone rings. Their caller ID comes up. Do you:
a) Smile because this will be a great call
b) Cringe and send it to voicemail
Think hard, talk much and you will build great teams.