At what point during the visit (if any) can you be 100% sure she is not LE? Is it after you put the $$$ down and she does not arrest you?  Or after you are both naked?  Would she ever arrest you as you are leaving?

And another related question is at what point is SHE 100% sure YOU are not LE?

Once she has your penis in her mouth, you are both pretty much assured that neither one of you are LE. But, as mrfisher would say: "I'm still not a lawyer." Might be a good question for the Legal Corner.

There are NO guarantees.  

Getting naked and actually committing any acts is no proof either.  I've read reports where they never got undressed and got arrested, others who got arrested while giving a BJ, and others where they went through with the whole act to the finish and still got arrested.

LE can twist anything into anything they want it to be.  They can also use civilians as CI's.  
Don't doubt it!  

Entrapment laws are non existent as well.  

But do you think a cop is going to screen you?  
Do you think a cop is going to have multiple reviews on multiple well known boards?  
Or do you think a cop is going to say, hey come right over I'm ready for you now!?!?
Always beware of a rush job and anything that is too easy.  The minute I feel rushed or pressured, I'm backing way off up and probably completely off.  
But there are NO Guarantees!

I keep looking to play stunt dick for LE, where I get to play on the taxpayers dime.  I've searched and searched, yet nothing.  Do you know how to find that job?  I'd work "hard"

I think scarlett said it well.  Check for reviews and see if she screens. But there are no guarantees.
All I can say regarding providers is: that is just one of several reasons why I screen. Personal safety being first.

That'll bring them flocking to your door.