This may seem like a no-brained but a visual examination which should go beyond just looking out for a scratched screen or loose back cover. You should be particularly concerned about liquid damage which leads to slow buildup of rust. Open the back cover and check the battery sticker for any signs of discoloration – specifically a yellow or orange hue. In case the owner has changed the battery, check any similar internal stickers for signs of rust. Also, check all the ports (USB and charging) for any signs of discoloration. These are metallic parts and any discoloration is an indicator of water damage.