Hi,
My son is 4 1/2 months old, and he is feed from the breast mainly (well always), however I did have to use a pump duing his 9 day stay at the NBICU!!
I always angled the pump on my breast and turned it on to slow, medium, then high, and get as much as I could from the pump, and my hand would squeeze on the sides of my breast!
I would do a slow motion of the squeeze, and I could feel my breast go limp in a way, from the heavy fullness feeling before pumping, to light and almost normal! LOL!!
I also could tell by the color of the milk, at first was light, then in the middle darker, then light agian near the end! Always white!!
Good luck and I hope I have helped!!