The main things to look for off the top I think are:
A Demo Reel.
Possibly a couple projects that fit the genre or style you're looking for.
Their attitude and how they hold themselves during the first conversation (are they asking questions, are they just agreeing with everything, are they polite, are they friendly, funny, ect. this is someone you're going to spend a lot of time with you better like them)
Once you've short listed people:
The connection between the Director and the DP (this is huge, if they clash heads in the first meeting then things are a slippery slope down)
Their camera and lighting knowledge. (what is going to be best for your story, do they have experience with that sort of package)
One I get a lot is the connection to great crew. I always get asked if I can bring along a gaffer/operator/assistant/ect. I can see from a producer stand point on smaller scale projects that this would be a huge plus not having to source out additional people on their own.