How to tell vacuum tube is gone bad??

Listening is the final and most important test.

Failing or aged tubes in general lose bass energy, the sound will tend to be harsh. If so, it is time to change them.

tube testing is often available at music stores for your guitar amp. if you've got used unit or used set of tubes, you can go and check there. in my area there are plenty of local small guitar shops and almost each have tube tester if you're not ready to get your personal. tube that went bad have a possibility to 'take away' other circuit elements such as capacitors or resistors in its power supply.

In my case a loud pop and a spark were a clue that I had tube failure