From what I understand, and I'm still learning as well so correct me if I'm wrong. A bird dog is someone who like you said finds profitable properties for wholesaler/investors and they are paid if the buyer closes on the deal. Typically a flat-rate that they have already aggreed upon somewhere between 500-whatever reasonable. Theres always that question how do I know if I will get paid. I'm sure theres forms for that type of stuff that an attorney can draw you up and I'm pretty sure theres a form on this website too. Your check doesn't come out of closing it comes from the investor/buyer. And no you do not need a license. You are not involved in the closing as a birddog. The idea is that if the investor values your relationship in providing him profitable properties than he'll defianately want to pay you to keep you bringing him deals.
Here's a contract on BP for birddogging. Remember to get it reviewed by an attorney in your state before using it.
http://www.biggerpockets.com/forms/bird-dog-agreement.pdf
Heres a link that I was checking out a while ago it really helped me.
http://forums.biggerpockets.com/viewtopic.php?t=3810
-Caesar