This depends on what the something is. So I will take it from that perspective. I know had enough of a particular food when I no longer have appetite for it and the mere sight of it irritates me then I'll know I've eaten it so much. If I stay away from it though for a long while I start to crave for of again.

I know I've had enough of a particular activity when I no longer enjoy doing it and can't get the usual satisfaction I usually get from it. If it's a particular job I discover I force myself to do it then I know I've had enough. If it's rest, I know I've had enough when I'm energized.

Basically, losing focus, Interest, joy or any form of entertainment indicates that one has had enough.