Is there a guide somewhere for what incorrect voltages indicate? It's interesting you can just tell areas to look at based on which voltages are off and by how much, I'd be interested in looking into understanding that more.
The way you learn that skill is to be a technician for a few years. Basically you look at a schematic for the device, or perhaps you can suss out how the design works by looking at it. Then you figure out what a voltage would change to if some part in the circuit was not connected properly or was shorted, and work from there.
For example, if the LEDs in the basic Crack blow (go open circuit), the tube does not conduct. So the plate voltage on the tube will go high. If you know the plates are pins 1 and 6, you know those are the pins that will read too high if the LEDs blow and go open circuit. Bear in mind that not properly soldering the LEDs could cause the same issue even if the LEDs are good. These are the details that you learn with experience.
Another example would be that you see a very low voltage at the + terminal of the electrolytic capacitors. That tells you that something downstream of them in the circuit is shorted and pulling so much current that it is loading the power supply down. So you look for wires that are shorted out to the chassis or the wrong terminal, or components that may have shorted internally (usually indicated by them looking very cooked).
