How to tell vacuum tube is gone bad??

Several ways:
1. Tube tester(the most efficient and fast)
2. Tap chassis of turned on amp and see if microphony is there
3. Swapping tubes from channel to channel

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.