Nexus 5 used mother board testing help.

I was looking for some tips on how to test the motherboard on a phone that has water damage (screen won't work). My phone's mother board is completely shot, and I found a guy selling a water damaged phone. I am going to go meet him today and I was wondering how would I test the board itself. He says the phone still turns on, notification light works, but the screen doesn't. Any tips?