It seems today that one has to spend considerably more hours researching a provider to determine if she is legitimate and safe than you are going spend with the provider.  TER is a good resource but not necessarily conclusive.  Here are some actions I have taken and questions I have.

Googling the Phone Number and/pr email address.  Of course, many providers use burner phones which are do not show up on a Google search.  Sometimes a telephone number or email address will show up on ads for different providers.  Does this mean it's a scam or that the providers go through an agency and use a common phone number/email address?

Googling photos.  Sometimes I will search a photo and find it used in ads in different states and under different names.  Of course some providers go on tour.  Also, people take providers' photos and use them on fake ads.  What's a guy to do?  And what does it mean if the photo does not show up anywhere else?

Any other ideas?

Is make yourself comfortable enough that it’s NOT a scam. Personally, I stay away from anyone whose picture or number shows up in multiple cities and ads. Not finding  a specific picture anywhere else isn’t a problem, not finding ANY pictures of her anywhere else is (for me at least).  
Often, if you don’t feel an ad is legit, it’s best to pass.