HTC One - What are the MUST DO things when you buy HTC One ?

Hi Guys

I'm gonna buy the HTC One in the next 2-3 days. When buying it, I wanna see if everything in the phone is okay or not, like seeing if the all pixels on the screen are working perfectly (no dead pixels), the headphones are working, the camera etc.

What are the REALLY NECESSARY things to check while buying the phone ? Also, What is that code, which you type in the dialer, which lets you see of there are any dead pixels on the screen or not (This is sort of a built-in screen test). Please help !