Hi,
It depends really. If the contols are out of the required envelope, the AP will engage BUT it will actually trip after a few secs. Happened a few times in a simulator, when we were out of trim on eng out approaches and go-arounds.
For us, the cue is waiting for the "click" to see if the AP has taken over control fully. The "click" is actually the locking device used to lokc the sidestick in position.
Hope this helps,
Cheers,
lion-g