I can tell when DD has had enough, more often than not these days she doesn't finish a bottle. When she has had enough she'll just play with the teat, it might be in her mouth and she might suck occasionally but will spit/dribble it out her mouth or chatter away with the teat in her mouth. She'll also push the bottle out of her mouth and turn her head away if I try and offer it again. Basically she will let me know if she isn't interested...if she's hungry she will drink without interruption, so if she's just playing around with it she isn't really hungry.