DateCheck, Preferred411, and RoomService2000.  These are the 3 big ones, don't know if there are any others.  Basically, they do the screening for you if you are a member of the site.  RS2K is more popular in the midwest, the other 2 are more widely accepted, based on what I have read.

Since many providers are reluctant to post their screening procedures in open forum, I suggest you contact a few experienced providers in your city for pointers on screening.

Swi

Utilize the Worldwide web & do your own homework.

Google search

First rule: If it feels fishy or you feel like something is inexplicably wrong, it probably is.
Second rule: Obtain as much of the following information as possible and use google and any boards to dig up any info possible on it. Name, phone number, email, any online handles and any aliases. Search each item individually and then all possible combinations, including any other info you gather. Often people will use a burner phone, email or name but if they have NO information at all, be weary.