Check the phone itself
Inspect the screen in bright light for cracks, dead areas and discoloration. Test the cameras, microphone, speakers, charging port, buttons and wireless connection. If battery health is shown in settings, check it rather than relying on a verbal estimate.
Make sure it can be yours
Ask the seller to remove their account and turn off any activation or device lock before you pay. Restart the phone and go through the setup far enough to confirm it does not ask for someone else's credentials. Compare the device identifier with the box or receipt if those are available.
Meet and pay carefully
Meet somewhere you can inspect the phone without being rushed. Ask what is included: charger, box, receipt or remaining warranty. Confirm the price and payment method before meeting, and keep a record of the transaction.