Someone introduced me to this method a few weeks ago. If you see an add on any site with a pic that seems as if it might be to good to be true, simply download the pic and then upload it to this site:
http://tineye.com
It's completely free. What it does, believe it or not, is track the pic on the web, it only takes about 15 seconds and it will bring up a list of all the other sites where that pic is used. If it shows up on a provider site, than means it could be real of course. But if it shows up on half a dozen "pay for porn" sites - then someone clearly downloaded it and used in in an ad. You would be surprised how many seemingly real pics turn up on amateur, girlfriend, asian , Russian teen sites. This is not fool-proof but it works most of the time.