To determine the authenticity of an old Gibson guitar, measure the guitar and compare it to the correct measurements found on Gibson.com, compare the headstock and the logo on the headstock to real Gibson guitars, and examine the wiring. Gibson's customer service can assist with legitimacy issues.
All of the pearl on Gibson guitars should be inlaid. If the Gibson guitar is a Les Paul model, the Les Paul model script should be written in cursive. If the truss rod cover has three screws, it is not an authentic Gibson guitar. It is a good idea to inspect the pickup and control cavities. If the routing is done poorly, or if the wiring is made of plastic or sloppily done, it is not an authentic Gibson guitar. The pickup cavity also should not be painted black.
It can be difficult for an amateur to determine the difference between a fake and a real Gibson guitar, so Gibson recommends contacting their customer service team for assistance. When possible, it is always a good idea to ask for an official Gibson owner's manual and warranty inspection card from the seller. Sometimes the price can be a strong indicator that a guitar is not authentic. Buying a guitar that is significantly under standard price is a serious risk. Gibson recommends buying Gibson guitars only from authorized dealers.