My only experience with a booster going bad was with a nearly new Z28 decades ago.
Brakes were normal starting out, failure was kinda sudden; pedal went nearly to the floor and it took A LOT of effort to stop.
Nothing really visible but turned out the canister halves hadn't been properly staked together and one rotated just enough.
That's 1 car out of about 90 that I've had, though maybe half have had manual brakes...

If you can apply a vacuum test to the booster, that'll tell you, but if it's total failure, it's more likely hydraulic.