In my opinion, you want to do as little as possible cleaning or wiping down. I keep my Commemoratives in the safe in their original boxes, and get them out about once per year to verify they're OK, and I simply spray the slide with a rust preventative oil. This oil is "stickier" than regular lube and is designed to stay where it gets sprayed.
Glocks originally come with something very similar on them (to keep them from rusting/degrading while sitting on shelves at the distributor and later the dealer. I like to see the copper-colored anti-seize compound still on the underside of the slide, with as little of it smeared from working the action as possible. It's OK with me if it's smeared a little, because as some point someone should have verified it's unloaded, but if it's wiped off or very displaced, it requires a lot closer look to verify whether it has been fired much. Also, one must be wary of people who buy the copper anti-seize and put it back after shooting 50 rounds through the gun, so this indicator is definitely not the be-all, end-all decider of whether a Glock is "used" or not. But, it is a quick way to get an idea.