Try to get some information on the agency. Go to reviews and see whether information exists on the agency. Put in the phone number and/or web URL for the agency and see what review information you come up with for it's ladies. If the reviews are bad or so-so, stay away. If the reviews are top rated, you have a find, go for it.

Gothicman is right on target.  The other issue even with a legit agency is that the providers aren't always on the up-and-up.  I had one who, after I shelled out $$$ for an out-of-town outcall, try to tell me that that just got her in the door.  Since the agency assured me 'total satisfaction', I called them right away in front of the provider.  The PO talked to the provider who was pissed.  After she hung up she told me she would do what the agency promised but not happily.  I replied that she could have the option of giving me back my $$$ minus a contribution for gas & time.  I wasn't in the mood after that.