My approach is reverse image search and if it hits on anything but another escort listing site then it's probably a fake. If you're asking how to do a reverse image search, google will describe it better than I can but I use Google Image Search and TinEye. And I use both because I find using just one or the other can hit but sometimes not both.
A fake photo of course doesn't necessarily mean a fake ad but it's a pretty big red flag in my books. I have no problem with advertising a look-alike photo, as long as its a fairly similar representation and its stated to be so.