Foremilk\hindmilk

B17
How do i tell if my baby is getting enough hindmilk? Also if he nurses on one side for 15+ minutes and falls asleep, should i start with the opposite breast at the next feeding? Im so lost i thought you should start on the one you left off on but my son isnt lasting that long and for the past 2 or 3 feeds has been on one breast only... Plz help this ftm....thank ypu