You can learn a lot about an STTNG by looking at the how clean the inside of the subways are under the PF.
Maybe play a virtual pin unless you are familiar with the rules. Go take the glass off and check all you can think of. Touch each switch and roll over and start game modes etc... make sure the machine is mostly working at least. If the machine turns on does it ask to go to test report or just boot up? Check boards in back box for obvious hacks and or battery acid damage from leaks. After that it is all condition and cleanliness vs asking price and value do not rush to buy just rush to go look at pins you are interested in. Deals are swept up quickly.

Good luck and welcome to the hobby