How do you know if the tube in a Peavey 6505+ isn't working?

My Peavey 6505+ head won't turn on when I plug it into the wall. I've checked the fuse and it is in working condition so what else could it be? I've been told that it's a possibility that the tubes arent wokring, so how could I check to see if they are working? Could I just look and say 'yep, those are broken' or would I need an expert?