When I exhaust my glycogen supply, after a big workout, within a few hours, I feel very tired and get the urge to want to sleep. If I don't eat some PWO carbs, I feel like a zombie until the next day - probably that's how long it takes to convert protein into glucose for me. Generally half a sweet potato is enough to avoid this effect. (Of course, I do also eat some meat as well, but if I only do meat, I still feel like a zombie.)

(I work out after a 16 hour IF, then I wait about an hour or so before eating again to allow for autophagy.)

i think the amount of glycogen stored in muscles, liver and blood can be approximated. how fast you burn through that will depend on the exercise being performed? perhaps you could identify which routines don't go well and plan to ingest a simple sugar during those workouts. this would answer the question of glycogen depletion being the problem if these routines improve.

the only other factor i can think of is how active you are prior to the workouts. if you begin the routine without full glycogen stores, you would run out sooner? maybe take note of that also?

if you eat low carb and are an efficient fat burner, once the glycogen is exhausted you should start burning fat. maybe someone more knowlegable could explain how that works?

You might be a bit tired until you get more fat-adapted, but the best way is to notice ketones on your breath or urine. This will also go away when you are adapted to burning ketones, but at first the ketones will spill out a bit more.

When I was truly glycogen depleted, my biceps lost about 3/4" in circumference. So if your muscles look a little deflated, that's a surefire sign.

Otherwise, I don't know if you are familiar with what it feels like to bonk, but that's how I know I have a glycogen depletion.