I'm quite surprised that Groupon could list fake watches on its site. Scary thought really. As for fake G-Shocks, those were quite easy to tell actually : scrutinised properly and you'd noticed that they write 1/1000 on the dial instead of Casio, and for those with special functionalities' module e.g. Master of G (Rangeman, Riseman, Mudman), the fakes have no such functionalities.