Have never done this but want too, Can you provide the best way to make sure that girl isn't undercover? Etc.
Thanks

Only one way that I know of - get yourself in the position where the provider will be alone with you.  LE would never truly allow their officer to be unprotected (I don't blame them).  Incall does not count because you will not know that you are not alone until it is too late.

keep your mouth shut about sex for money and don't mention sex at all until both of you have undressed.  For legal reasons, you could be paying her to just cuddle with you while you talk your problems out