Sign up for P411...awesome tool!
www.preferred411.com
Some ask for provider references, as in the guy will give you some names and emails of girls they have seen! Then you contact them and ask them if they have seen him, is he safe, does he give correct donation...whatever you want to know! But make sure you search the girls name or email too, in the search section here or Google and make sure they are legit with several reviews or a p411 account!
You can also just look for reviews of well know providers!
You can ask for work verification also!
Whatever and however you decide you want to screen, make sure you feel 100% safe with the method! These are just some methods of screening...there are plenty more I hope some ladies share!