How did you know your supply was dropping?

Bub is 4 months next week and EBF. Happy and shocked I made it this far! I really love breastfeeding and the thought of it ending really makes me sad. I never had a goal of how far to get but now I'm at 4months I just want to keep going. I feel like my supply is dropping though. I pump once a day for stimulation after bub has fed and in the last week I missed a few pumps. Now when I pump what I collect isn't even worth saving. How did you know your supply was dropping and when to switch to formula?