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Is to have potential client give you references from a couple of ladies he has recently seen, call them and get their thoughts.
Unless the guy is a newbie, that should work well.
I think most providers are willing to give referrals.

You should ask clients to be members of a screening agency such as P411, Datecheck, or Roomservice.  

In addition, ask the person for their true name and address, or place of work.

These or combinations of these are the most common way to screen.

Also there is a white list for clients on TER that is often used.

provider only board. Most ladies have secret methods they don't want to share with hobbyists.

They won't let me on the PO board! I've got 11 reviews... and they still won't let me! So, idk wtf! lol. Thanks for the info though!!

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The written rules say you need 3 reviews to get onto the providers only board, but they also want you to have a posting history. Try posting more on your regional board or any of the general boards, for a month or so, then ask for admittance to the PO board again.

In the meantime, some providers may be willing to give you some advice via PM or email. As kk said there are screening methods that providers don't like to share on the public boards.

However there are many common methods that are well known as mrf and joe mentioned.
1) References from other providers
2) Memberships and "oks" from verification sites.
3) real names and work info
4) TER whitelists
5) google (names, board handles, phone numbers, email addresses)
6) blacklist sites
7) gut feelings

Most ladies will use one or more of the above. Every lady is different in how she screens, and what she accepts or feels comfortable with, and every hobbyist is different in what info he is willing to provide.

Figure out what works for you and don't compromise