The nut on a fake usually looks more like an Epiphone nut. It's thicker than a Gibson. Gibsons are built with a head-cap that lays against the back of the nut. (the left side) Epiphones and the fakes have a thicker nut that lays on top, with nothing against the back of it. UPDATE: I recently had my hands on one of these that had a head-cap more like a real Gibson. Removing the truss rod cover revealed a couple of obvious details, however. The cover itself was hand made, very poorly. Under the cover you find an allen (hex) wrench truss rod in a messy hole. Gibsons should have a rod that protrudes outward so a nut driver can be slipped over it. If it's just a hole in there, that's wrong.
If you're buying a Gibson from someone online or on Craigslist, you need to look carefully at this part. They may have upgraded everything else on the guitar, but this part can't be hidden.
The bridge and tailpiece usually have sharp edges under the chrome plating. If you run your finger over the edge, it should feel smooth. If it has a harsh edge, it's not a Gibson part. The bridge usually has 6mm posts you would see on an import. Gibson doesn't use these. They use the skinny posts that don't have a slot in them for adjustment.
If you're buying an expensive guitar, the seller should take no issue with you opening the control cavity to look inside. You should see braided wires, large pots (that usually say Gibson or possibly CTS on older models). The routes should be clean and not left with burrs and sawdust inside. If you see a rat's nest of red and blue and green wires, then it's not real.
Finally, my advice to anyone buying their first Gibson: Go to an authorized dealer of Gibson guitars. Play a couple of them. Get a feel for what they're supposed to be. Then you'll have a little ammunition to keep you from making a terrible mistake. If buying from a private seller, meet at their home. If they want to meet at a 7/11 at night, don't do it. Have them meet you at my shop or bring someone else who knows for sure if it's real or not.
We've seen a handful of people who had no idea their Gibson was fake. Tracking down the seller later will be impossible. If the deal sounds too good to be true, it just might be.