How can you tell when tubes are shot?

I was advised to look between the pins and if there were dark spots between them then it was time to change out. I don't know how valid this info is, but I did have some Bugle Boys that sounded soft. I pulled them out turned them over and lo and behold dark spots between the pins. Go figure. Best of luck on your search.

When I was in the Air Force, I worked on a few systems on
B-52's that had tubes..and we had tube testors..but they
don't tell the whole story. A Ferrari and Porsche could
look almost identical on paper..or in Road & Track test
results...but still "feel" and drive very different from
a driver's perspective.... and an old tube could still look
to be good, when indeed it would sound poor, or two of
the same model tube from two different makers could both
measure the same..but sound very different...IE: trust your
ears.

when i need to know if a 9v battery is still good i touch my tongue to it. i'm guessing you could do the same to the tubes while they're in the amp. i'm going to test mine right now.

WARNING WARNING WARNING

ignore previous post. ignore previous post.

Guess the girlfriend wasn't around Kubla?