It's something you just kind of learn. Your breasts will probably never be totally empty, as you will produce some milk in response to suckling. But the "empty" breast will feel kind of "deflated" and be smaller than when you started. It's hard to explain, but you just get a sense of what a heavy, full breast feels like and what a light, "empty" breast feels like. But don't worry about it too much. I just let my dd nurse until she's finished (she'll come off the breast or keep my nipple in her mouth but not suck), then offer the other. I can still get milk out of the "empty" breast, but we've never had any problems with this way of nursing.
HTH!
