If you are going to buy a second hand/ used phone, the best practice to check if it is working properly or not is to check its every feature. Checking hardware is not possible in limited time. So, when buying a second hand phone always download an app called "Phone Test (hardware info)" available on Playstore.
Once the app is installed you can test following features:
1) Sound Test
2) Vibrate Test
3) Brightness test
4) Touch test
5) Multi-touch test
6) Camera test
7) Sensor Test
8) Compass Test
9) WiFi Test
10) Bluetooth Test
11) Microphone test
12) LCD Test
These tests can also be performed after dialing a certain code, but with the help of an app you don't have to remember those codes.
All these tests are performed in the video.
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