You never can, just because someone has paperwork they bought it doesn't mean they never routed it either. As folks have said, it really comes down to condition. However, some tell-tale things to look for are extra bolt holes for coin door hasps (but some games came with them from the factory), matching serial #'s on the boards with no sign of operator hacks, lack of fingerprints and dirt under the game by the power switch, non-rusted feet levelers, "burn" marks around the light bulbs in the head from being left on 24/7, pretty pristine sliders on the back game rails, and the condition of the lockdown bar (hopefully not many drinks spilled on a home game). However, don't let the coin mechs fool you. They should be a lot cleaner, but I and several friends like the nostalgia of putting quarters in so HUO does not necessarily mean that a quarter never passed through it.