It announces it with a quiet little “your sentry is under attack” from NOVA, it also flashes at the same time. The problem is both are very easily overlooked and should be more prominent, AND the sentry will do BOTH while not actually under attack.
I mean, maybe it’s some stray bullet going across map to activate it when this happens, but it’s literally not enough damage to be able to even see that the shields are down at all, and it will go haywire. It’s not reliable.
This results in the first few times it happens you rushing over and looking for the enemy damaging it… then, you start to assume that if you’re pushed up into the enemy’s base it’s probably just a fluke. That’s a failing game-mechanic that does not accurately portray the state of the game.