I knew because a) he always seemed hungry after he 'ate', b) very little swallowing when he did eat, and c) he wasn't gaining enough weight. Then I rented a scale and verified it with weighed feedings.
IMO, this board causes people to worry about supply a little more than is necessary.
Not to say that low supply isn't a real issue that some women battle - it is and my hat is off to all the efforts some women put into to get their LOs as much BM as possible. But for the most part, if you're feeding on demand, your supply is going to be fine.
You can tell because LO will seem satisfied for a at least some time after a feeding (they may want to eat 45 min later, that's just cluster feeding and not low supply), LO is gaining weight, and diapers are okay.
A couple of "bad days" nursing shouldn't hurt you. In my mind, that's one of the reasons our bodies are set up to take a little time to respond to changes in demand. During growth spurts it's a pain b/c it takes a few days for us to catch up with the new demand. But the good news is it's not like one bad day will cause your body to think "oh okay I'll just make half as much milk tomorrow." KWIM?