The lug is stamped either ".750" or "18K". The links are thinner, one-piece construction, and have a different cross-section than the goldfilled ones. The expansion (removable) links are held in place by screws, not springpins, and the bracelet is attached to the case with screws, also. There are differences in the clasp, too, the most noticeable ones being that it is shorter, and the Omega symbol is a raised, added piece of gold, rather than being embossed on the surface. Finally, there is -something- about the look of 18K gold...
Addendum...notice that the screws attaching the bracelet to the case are stainless steel (presumably for strength), and not gold. I replaced the ones on mine with custom, home-made 18K screws, just for the sake of appearance (the watch will never be worn again, so the stronger stainless screws are not really needed any more...it looks really nice, too).