In this amp repair video, I'm basically showing how you can take a drumstick and probe around to see if anything is loose on the board. Also give a light friction test to tubes. If you have a bad tube, sometimes a light tap can get it to spit or malfunction.
You have to keep in mind that not all customers are going to want all the tubes changed. So you find the bad ones by different means. Of course you can dig in and take voltage measurements and such to diagnose a problem but sometimes it's very obvious what the problem is if you use your eyes, ears and nose.
Also on the scope you can see noise as the waveform breaks up on the screen.
In other videos I will get more in depth with signal injection and analysis of the circuits.