1. Activation Lock
If Find My is still enabled by the previous owner, the phone cannot be set up by anyone else. There is no workaround, and support cannot remove it for you.
The phone should arrive at the “Hello” setup screen and never ask for someone else's Apple Account password.
2. Carrier lock and IMEI status
A locked phone only works on one network. Ask for the IMEI (Settings → General → About, or dial *#06#) and check it before money changes hands — carriers and third-party services can tell you whether a phone is locked or reported lost or stolen.
3. Battery health
Settings → Battery → Battery Health & Charging shows Maximum Capacity. The same screen flags a battery that is not genuine, which affects both the price and what the phone will do under load.
4. Parts and service history
Settings → General → About lists parts that have been replaced and whether they were genuine. A replaced screen is not automatically a problem, but an undisclosed one tells you something about the seller.
5. The physical tests
- Every camera, front and back, including video.
- Face ID or Touch ID enrolment.
- Both speakers and both microphones, via a voice memo.
- The whole screen for touch response and discolouration.
- Charging on a cable, and wirelessly if the model supports it.
6. How long it will keep getting updates
Support length is tied to the model. An older phone in perfect condition may still have fewer supported years left than a newer one with a scratch, and updates are what keep a phone safe to use.