Listening to your body vs totally blowing it

So, I'm not perfect. Big shocker, huh? :noway:

There are days when I don't eat smart and either use up my calories, or don't eat high enough quality foods that don't keep me full for long enough. Of course those are all part of the learning curve, but there are days when I either don't eat smart/right or I'm a little more active than usual and end up feeling hungry, or feel hungry the next day. You know, that low blood sugar, shaky, weak, lightheaded feeling? Yeah, that.

So obviously my body is telling me to eat more. That's cool. It obviously needs it. And one day here and there doesn't undo everything, as long as the "extra" food is good quality, healthy food.

But what's the line between just needing to eat better, and needing to eat more? There are days I can eat 1000 calories and feel full, satisfied, healthy and happy. Then there are days that I can eat 1500 (not counting exercise) and still feel hungry. I guess my question is, how do you know if you're just not eating enough overall, or not eating the right foods to feel satisfied?