Look at the truss rod cover. Simple as that.
Classic example of a fake Les Paul with Gibson on the truss rod cover
If the truss rod cover has three screws in it, (apart from the ’67 Flying V, Firebirds and Sonex) your looking at a counterfeit, if it has Gibson written on it as well then it’s possibly an Epiphone, (if you’re lucky), but what it definitely isn’t, is a Gibson. Gibson Baldwin manufacture student model guitars where you may find truss rod covers with three screws, as well as bolt on necks, these are not to be confused with the high end Gibson USA guitars.(pictures from Gibson.com)
