when do i know she's finished/had enough to drink??

[yummy*mummy]

hi, my baby is 3 weeks old and im new to this b/feeding thing as i didnt with my first. How do i know she's finished feeding? is it when she falls asleep or if she's pulling at the breast? i usually feed for up to an hour and she usually goes the 3 hrs before needing another feed.
also how do i know i've drained my breast? when she starts pulling at my breast does that mean it hasnt got much milk left and should i then offer the other side??
im a bit confused
thanks