Good evening! I have a question on how to find out if the baby is getting…

Good evening! I have a question on how to find out if the baby is getting enough while breastfeeding, as it constantly opens its mouth and sucks for 15 minutes and falls asleep immediately after being put in bed, it starts crying normally only after an hour or half an hour, but has gained only 600 grams in a month and sometimes goes 2-3 days without pooping ?