1. Battery Status
Settings > Battery > Battery Health. 90-100% excellent, 80-89% good, below 80% noticeable degradation.
2. Check iCloud
Settings > [Name]. If it asks for another account's password, do not buy until it's resolved.
3. Screen Review
Display a full white and black screen to check for dead pixels or spots.
4. Cameras
Test normal photo, zoom, video, front camera, and flash.
5. Face ID or Touch ID
Set up and verify it responds correctly.
6. Speakers and Microphone
Play music, record a voice memo, and if possible, make a test call.
7. Connectivity
WiFi, Bluetooth, and mobile signal.
8. Serial Number
Verify the exact model and if it's locked on checkcoverage.apple.com.