I recently did that on one of my cherished Sano 160R 1x12's. I lost a nice premp tube of all things but the main circuit was intact and I ran it like an idiot for about a full minute wondering why I had no sound
Anyway, it hummed like a banshee but again, it was just a preamp tube I shorted. Unfortunetaly, that tube was a NIB 70's original Tesla ECC803 of which I only had 7. I only have 6 now
Like Swarty said, if something failed, there will be no doubt.
I guess the real answer is that if it doesn't sound any different, everything is the same. Usually you'll get a hum when running without a load damages something whether that's an output tranformer or a tube.
Well that raises a new learning point for me then. So you're saying that an OT without a load would theoretically last longer before damage with no extra current going through it such as just sitting there with the guitar not played or no guitar plugged in at all versus hitting power chords?
It was always my impression that OT would go faster than they apparently do since I've done that probably 4 times in my life now.
The magnitude of the flyback voltage is proportional to the signal applied to the OT. With no signal applied (guitar unplugged, or volume all the way down), the the source signal is basically noise and (usually) low level, so the flyback would tend to be smaller. Obviously the opposite applies for the power chord.
If you've got an amp that has a high noise floor, or that occasionally spits and pops (finicky tube, aging plate load, etc.) then the noise, spits and pops can be more than enough to generate a monstrous flyback spike that will take out the power section.
In a quick stage setup once I was using 2 bass amps, an Ampeg B-15 and B18, stacked. I neglected to connect the B-18's speaker since the head was on the floor and started the set. When I realized that I was getting no sound from the B-18 I quickly plugged in, but I'd bet it was 3-4 minutes of playing with no speaker load. The amp survived with no damage.
I've seen the opposite also, once quick chord without a load and the amp blows.
What I never have seen is an amp that is injured and blows later. It's either immediate and obvious or never.
Bottom line, be careful. Johnny's been lucky so far.