How to ensure that the digitizer is faulty?

My HTC Evo 3D sometimes doesn't react when i press certain areas of the screen. I noticed it happens mostly on the part where the letter 'e' is, when i'm holding the phone in landscape mode. So, i did a factory reset, wiped out SD card as well during the factory reset, and the problem still persists. It sometimes happens for a certain period of time, and it starts to work after a while. I did a line drawing test, and on some places it doesn't draw the line correctly (it is continuos but it's drawn with sharp angles, i don't know how to explain it better). I read somewhere it is most probably a hardware problem, with a digitizer. Is there any way to be sure, or is it best to take it to a repair shop?