How do you know when you’re empty

How do you know when your LO has emptied your breast and is ready to switch sides? My son is a bit of a lazy murder (or so I thought?) and doesn’t nurse for very long. However I have sometimes found that if I switch sides he will drink again after having given up on the other breast. It doesn’t seem like he is nursing on one side long enough to have drained it but it still interested in the other side, but I want to make sure he is getting the fatty milk and completely draining me. How do I know?