How to tell if I'm losing my supply?

Cici • Mama and Etsy shop owner! My etsy shop is SerenadeBaby
Help! I think my supply might be dropping. LO will keep latching and sometimes will eat for a bit, just to start crying and crying again. He is hungry, I will give him a bottle (previously expressed breastmilk) and he will take it right up, but seems to get frustrated on the breast 😕. Could this mean my supply is dropping?