They will usually say. If they haven't, clothed along with a disguised face one (think mask / dark glasses) should be fine. Consider 'watermarking' them to ensure they can't be used without your permission.
What sort of pictures do they have on the site? What can you find out about them online? (Particularly, how easy are they to find?) Where do they advertise? What do reviews about their staff say?
Think very, very carefully before allowing your face (or any personal tattoos etc) to be seen with anything connected with escorting - once you do, it will be out there forever.
What you ideally want to do is to talk to some of the people already working for them.
Oh, and if the owners even hint at a freebie 'test', it's time to strike them off the list.