You will know a down and an up by the unmistakable feeling.
Mighty Sun Tzu (whom I call Sun affectionately)
posted recently that he felt fantastic at the end of his water fast. Many of us here have felt both "low" periods, and "high" ones during a water fast. (They are utterly different than master cleanses, which I have found to be more difficult) The latter (the up times) are experienced as a feeling of well-being. Sometimes these can be blissful, or happy feelings with energy (and it's important not to overextend oneself when the good feelings come), but in any case they are very different in tenor from "low" periods.
you will know by your own experience is all I can honestly say. This assumes that people are not interfering with the WF by taking other things, like supplements or anything besides water,which can and may well contribute to an ongoing gnawing "hunger" or sense of depletion.
As to the question about reserves, and starvation and how the body spares itself--you would feel overwhelming unstoppable, continuous hunger if you reached that critical point. The keyword is *continuous*. It is a hunger that demands satiation. The sort of hunger that has led people to eat *anything*. It's possible your mineral reserves are low, (which is a common occurrence with the nutrient-deficient SAD and junk diets). This could cause you to fantasize a lot about good food, but the thing of wanting to eat whenever you see family members eat--this is more mental/emotional/social. Yes, there's no denying it's strong--this is what we have built whole societies around: food.