I've burned out twice in my life. You'll know it when it happens. It isn't just being tired. It's stopping caring what happens if you just quit on everything because you simply don't care anymore.
The trick is to learn how to do less at your job without getting fired. Learn how to cut corners. Care less about your job without losing your job. If your job can't provide you with a reasonable work environment and is affecting your physical and mental health, then you shouldn't feel guilty cutting corners to maintain that health.