You can never be completely sure but aint that half the fun of it????

Some of my warning signs/suggestions:
-look for someone who is reviewed
-stay away from ads like Craigs List, Backpage, etc...especially for a provider who hasn't been reviewed
-make sure she asks for verification info and actually checks up on you
-if you contact a provider and she says "oh yeah...come on over now"
-check your local boards, if there is a sting going on it will normally hit the boards within a very short time

I've never heard of a reviewed provider being part of a sting or LE somehow posing as a reviewed provider but in general LE stings are relatively rare...

There are no guarantees but if you stick with well reviewed providers, more than 6 or 7 reviews posted from established reviewers, you have little to worry about.

Check to make sure she has plenty of reviews. With this hobby you never know so just make sure you stick to the values of well reviewed providers and even better if they post quite frequently on your local board. Another thing try and stay away from clist hope that helps.