When I was breastfeeding I would know it was fairly empty when I couldn't manually extract any milk.
From what I understand your body will ensure that the baby is getting hind milk. For instance...if you switch sides your body will know to only produce enough of the three types of milk so that the baby will get to the hind milk at each breast each time. If you only feed on one side it will adjust. I am sure there is someone who is more of an expert than I. I was told this info by a le leche person when I was bf.