The YouTube videos should be OK.

The signs you should be looking for are simple.

Anytime that you think you missed something , you brain is finished for the time being.

Anytime you need to reread a line your brain is fatigued.

Any time you sense a need to refocus visually, your brain is fatigued.

Any time you sense you zoned out, your brain is fatigued.

And so on.

Sometimes, like at work, all you need to do is close you eyes for a few minutes, sometimes, just a few seconds, to get your brain back on track.

It is a skill that you will need to develop by trial and error. In the beginning, it will take longer to recover. Once you learn to recognize the fuzzy brain, you can stop sooner and recover quicker.

Taking a eyes closed break at work during coffee breaks will go a long way to helping you make it through the day. Even better if the eyes closed break is where there is minimal talking. Shorter but more frequent breaks is better than longer and less frequent breaks.

Hope this helps.

My best to you.