I see a lot of people advertise that a game is 100 percent original. Specifically I am interested in how tell if the cabinet is original.
Pretty much what WaRpED said, but it will be pretty obvious. Does it look 20+ years old? Are there signs of some sort of abuse (scratches, cig burns, peeled labels, gum stuck in places you didn't know existed?)
Some of the above is a bit tongue-in-cheek, but the big thing is that the manufacturers name, logos, copyrights, serial numbers, etc. should be all over the inside of the machine. Bootlegs were a big problem, so they went out of their ways to make it obvious.
You also want to do as much research on the game in question as possible. If the game had WICOs in them, and the one you are looking at has some Supers (or Chinese knock-offs) then not only is it not 100% original, you can probably assume a lot more of it isn't, or that the seller probably doesn't really know and isn't interested in finding out.
RandyT