Always test Samsung Android devices before buy it (only valid for shop purchase though). Yes, this applies to Samsung flagship Galaxy S4 too. Samsung makes a great tool for them to check the hardware quality at test development stage. And you can use it to test too. The application is called FactoryKeystring. Basically, it covers all the hardware function tests for Galaxy S4. You can check 1080p Full HD Super AMOLED screen with Red, Green, Blue, Receiver (white) and Vibration (black) colour. Make sure that there is no dead or bright pixel. That’s the first thing I will do. I really hate dead pixel as it is very annoying. There are other tests like Dimming, Touch and TSP Hovering to test the screen function as well. Front and back camera can be tested with Mega cam and Front cam. Both speakers’ tests are included too. Sleep, Sub key, LED, IR LED, LOW FREQUENCY, Barcode Emulator and SensorHub Test are the rest of the tests. Test all of them if you can.