Start with the screen and body
Look at the display under bright light and run the touch test across every edge. Check for dead zones, flickering, burn-in, cracks, lifted glass and frame damage. A clean screen with a faulty touch layer can be more expensive to repair than it first appears.
Test cameras, speakers and microphones
Open each camera, check focus and inspect the lens glass for haze or scratches. Play several audio tones, record a short phrase and listen for buzzing, muffled sound or rattling. Test the earpiece and loudspeaker separately when possible.
Check battery and connectivity
Review battery health in the phone's own Settings app because browsers cannot expose every battery metric. Connect to Wi-Fi, pair Bluetooth, test charging, confirm mobile data and make a call if a SIM is available.
Confirm ownership and history
Ask the seller to remove their account and disable activation locks before you pay. Confirm the IMEI matches the device and packaging, check blacklist status through an appropriate carrier or service, and look for liquid-damage indicators.