Breastfeeding

Hi Mummies.. I am latching my 3 months old lo on demand and pump after every session to keep my supply. However I realize that I still get about 80-100mls when I pump after each nursing session. I am a low to average supply mum so it makes me wonder if my lo is getting enough milk during breastfeeding or is she just sucking but not drinking. She can latch for more than 45 mins each session. So far, diaper output is very good. But weight gain is not very good. How else to tell if baby getting enough milk.. Please advise.. TIA ?