I always wondered this, too. So many of my friends wean because "the milk was all gone". After about 7-8 months, I'm not really leaking, and I feel like I can't tell a difference between the way my breasts feel before and after nursing.
I've nursed both of mine until they just self weaned. Both at 18 months, both coincidentally when I was exactly 20 weeks pregnant. I never got engorged, nor did I feel any different when they weaned, so I figured the milk was gone.
I did ask my last 18 month old if there was milk, but he looked at me like I was speaking Greek.

(he doesn't answer yes or no yet).