Some say true hunger is more in the throat than the stomach. Feelings from the stomach could be a number of things so they aren't usually a sign of hunger. I've also seen true hunger described as an overpowering desire for a single natural food. A pleasant feeling though, not the chocolate type craving where you'll kill your best friend to get some. The body is really hungry which doesn't often happen nowadays and the difference is that it 'craves' or chooses a food that will nourish it and not be harmful to it rather than a food that it has been addicted to either physically mentally or emotionally.
The amount of stores you have in your body will govern the length of time you can fast before hunger sets in. That isn't always so easy to tell by looking at a person. I remember a work colleague who was supermodel bone thin but who it was discovered had a tumour the size of a grapefruit in her belly! I would imagine that even though she was skinny the body could have cannibalised that huge mass of protein for quite a while before needing more fuel. So the only real way to find out is to do it.