To quote an eBay discussion group.
"Always question the photos. Ask for the movement shot. And stay away from the replicated serial numbers H0035/2000 on PAM111 and
H0117/1000 on the PAM177 and 217."
And from TZ-UK, a very good post by "Junior".
"Caseback numbers are usually a good pointer. For some reason, most fake Panerai (of all types and models) are number 117 - either 117/1500, 117/1000, or 117/500 - goodness knows why. If it's a 183, it will have an exhibition back, so the movement can be seen; it should be a calibre OP X1, which is a tweaked ETA 6497/2 (
http://www.paneraisource.com/details/OPXI.html). The opposite end of the dagger pointer at the swan's neck regulator has the typical ETA single brass screw, whereas the Asian clone movements have two silver screws. I'm sure there are several other indicators, but these might be a good place to start looking."