You don't.

Think about this:  how do you know you can trust any stranger?  You have to rely on instinct, right?

With providers, do research.  If she has many strong reviews, you stand a good shot at NOT getting busted...but even then there are no guarantees.

It's a risky hobby.  You only have two choices to pursue it:  go for it and take the risk, or don't play the game and take no risk.

Restrict yourself to seeing ladies who have several good reviews here on TER.  Also, when reading a lady's reviews, research the other reviews written by each reviewer.  This not only goes a long way toward ensuring the provider is legitimate (or illegitimate, depending on your point of view), but also helps you qualify the validity of her reviews:  If you find a reviewer who consistently gives 9s and 10s on performance, irrespective of whom he reviews or what the lady earned from her other reviewers, you may choose to take that reviewer's comments with a grain of salt.

It all comes down to research, research and more research.  Imagine what this was like before services like TER.  For a small VIP membership fee, you not only have immediate access to all this information, but you have access to insightful and thought-provoking discourse written by the likes of yours truely (actually, I think the insightful discourse crap is free [grin]).