What to check for in used iphone?

I'm going to buy a used iphone off of craigslist and I want to know what type of things I should check for before I buy it.

MAke sure the battery is still good. Usually old iphones have a bad battery and need to be charged more often.

Battery should be checked like the other guy stated. I'm not sure if you are meeting up with the person for a purchase so I don't know what I should recommend you.

If you are meeting up with the person, you should check the back and front to see if there are scratches or cracks. You should also test out the charging and headphone jack to see if it works well. The speakers, mute switch, volume buttons should also be tested.

That's about it, if you check all these things and it's fine, then you should be fine.

P.S You should turn on the screen and check if there are dead pixels or not, a way to check is taking a screenshot of something that is completely white and view the image without any borders, and other things covering the screen. Then you can see if a pixel is not working.