How did you know your supply dropped?

I’m EBF currently, although I just started one pump daily to start building up a stash because I’ll be going back to work in 2 months. My babe has been growing great and having plenty of wet/poopy diapers. My breasts are not always as full feeling as they used to which makes me wonder if my supply has changed at all, although I have no other reason to think that it dropped. How did you discover your supply dropped? What were the signs besides weight loss for your baby?