I assume you are referring to ads with providers that don't have reviews, so with those...

I personally avoid ads that have very young looking provider pics (i.e. those that say they're 18+ but look younger) - I think LE really wants to nab the johns looking for the youngins... which they should in my opinion.

I gravitate toward safe-by-association ads (i.e. ads that I can link to other providers I've seen by location, etc)

I normally see only massage providers so my risk is a bit lower since I can always refuse the extra services if it does not feel right (as long as I didn't implicate my intentions in the appointment setup phase).  This approach does not work for escorts.

I agree that the diamond in the rough hunt is appealing, but it's risky business and is probably just a matter of time before you run into LE.  This is why 99% of the time I see reviewed/trusted providers

If there was some way to know if an ad is part of a sting operation, everyone would know about it, police wouldn't bother with stings, and you wouldn't be asking this question.

Personally I'd just stay away from unreviewed BP providers altogether. Nothing wrong with waiting until a lady has a few reviews. Easier to stay away from scams that way too.