Always, always scrutinize the game to check for any damage to the cabinet. Yes, average wear-and-tear is completely normal and expected. However, when buyign your first game, it could be easy to overlook more serious damage such as severe swelling (from exposure to water). Also, check to make sure the cabinet artwork is at least in decent shape... replacing that can be very costly and is time consuming to track down the pieces of artwork.
Of course, make sure the game is complete. It would kind of suck to buy the game just to realize a vital part is missing (boardset, wiring, etc). It never hurts to get a look inside of the cabinet before you purchase it. That is, assuming the game is not currently working.
If the control aren't working 100%, the marquee light is not working, the metal harware and brackets are kinda rusty, etc, don't worry too much. Stuff like that can (generally) be easily fixed. However, use even little things like that to your advantage in order to get a lower price. However, don't point out every little flaw... this could be quite irritating. Simply mention that fixing up those things is going to cost you extra money.
Last, but certainly not least, get a price in your mind and stick with it. It is easy to get lost in the moment and over-pay for a game you would later regret purchasing for that price. Think of a reasonable value for it (what you are willing to pay) and don't gp over that price.