I agree with thirdeye; you'll know when you'vel earned something when it just comes to you as you're drawing. You'll try to draw a torso and suddenly you can picture exactly how the pectoral muscles would look in that position. It just...pops in there, since you've looked at so much reference. It's becomes natural.
I don't know if this has been mentioned, but here's a good site for getting started with shifting one's mindset in regards to drawing heads: