How do I know..

How do I know if my bf baby is getting full? She latches for at least 15 min and then falls asleep and doesn't want to latch onto the other side. Then she's up again in an hour or two and she nurses for 15 min each time. I have a pretty good supply. Once she gets the milk flowing, she's getting a lot but I'm just not sure if she's getting full or not! She's almost 3 weeks and VERY gassy if that helps.