You should make sure it flies properly. If you're a seasoned Phantom flyer, that should be easy to determine if you take it for a test flight. If you're not, then it would probably be better to have the owner demonstrate it.
You can see how many times a battery has been charged in DJI GO. There are flight logs stored on the Phantom's internal memory card, but they likely won't make sense unless you know how to read flight logs.
This is great when you sell locally, but many sell online and ship their phantoms. In a case like that you'd have to either ask if they have any video's they can send of their bird and/or ask as many questions as you can. I purchased mine from the classifieds here and the seller was honest and answered all questions I had and even gave me his number in case I had any issues. I've never had a problem though buying online.