Carbonfiberboy 

Well, one can tell one's low on glycogen if one just can't move the bike normally - assuming there's no other good reason, like lack of recent exercise or really sore muscles on the bike. Glycogen synthesis normally varies with quantity of carbs ingested, though when on a carb-restricted diet, gluconeogenesis will occur to some extent. As that banana's guy says, "Carb the F up!"

OTOH, if you want to know how one can tell exactly how many calories/kg of glycogen, both muscle and liver, one has on board, I don't think one can without doing some very specific and rather odd workout(s). There was that old thing about glycogen depletion as part of a glycogen maxing effort before an event, but even then, one wouldn't really know, too much other stuff gets in the way. However, since some of us do have body comp scales, one could look at one's weight and body comp which one had before an exceptionally good ride and try to duplicate that. And in fact, that's what one does. I think that's what the OP was wishing to discuss.