The knock offs have gotten really good nowdays. Before all you had to do was watch the movement of the second hand. A lot of the high end watches have specific face and band design for each model. If you can get ahold of a catalog of the brand you found, look for a picture of it. If you find one with the same face, but it doesn't match the band or maybe the color combination you have can't be found, then it is probably a fake. A place that sells high end watches could probably tell if its real or fake as soon as you walk into the store.