How to tell if my supply is decreasing?

Hi mummies need your advice but also a bit of a rant. My mother and I are having some disagreements on feeding my 4 month old baby. Baby is currently exclusively latched plus I only pump once in a while for emergency uses. As far as I know, breastfeeding is going well since baby latches well and gains weight well (a bit bigger than average). But my mother says my supply is not enough for baby and need to top up formula. I'm not against formula feeding however I don't see a need as breastfeeding is going well, and I don't know whether baby will react well to formula. I'm resisting but my mother keeps complaining and she keeps feeding baby my pumped stash which I want to save :( even though I said I can just directly latch. The reasons she claims my supply is low are: 1) my boobs look soft and flabby (I must agree this is true, my boobs used to feel much heavier and firmer in earlier days but now not so much) but I'm not sure if it's an indication that my supply is low? And 2) because baby is very fussy lately even after feeding, but I feel fussiness can be caused by many reasons not necessarily because of hunger. Every time baby cries my mother would make remarks that baby is hungry and my empty boobs are not enough to make her full which makes me feel so disheartened and sad. I don't know who is right anymore, I really can't tell if my supply has gone down, the only fact I do know is baby is still growing well and chubby, although quite fussy sometimes. When I do pump I'd get about 40-80ml per boob (I usually pump about 1 hour after feeding baby). What do mummies think? #pleasehelp #advicepls #breastfeeding