I think you know when the mania is bad enough - like after being crazy manic for about 3 months and convinced that we lived in a 3 dimensional world only I had the power to control I ended up somewhere on the street outside my compound and the receptionist called my mom who immediately took me to the hospital and then the doctor saw me and decided I had to stay in the hospital (another branch) for a month.

Every time I'm really manic it's very easy to tell, I stay up all night, find crazy solutions to problems I invented myself or suddenly I'll believe I know the reason for why we exist etc etc and I won't shut up about it, I'll talk to anyone and everyone I meet and spend hours trying to convince them and get SUPER frustrated because of course they dont believe me (once I even went to the local university to talk to professors there about my theory of life) and usually the people that encounter me call the psych ward or my parents. Just because I am sooooooo persistent when I'm manic, I could talk to you about why sheep are blue and get sooooo worked up about it until I feel like I'm about to explode from my own body.

And from going to the docs after these manic episodes it automatically takes care of the depressed / low time that comes after cuz you get mood stabilizers etc etc. Like the docs HAVE to pay attention if you've been manic - and they usually find that you are bipolar and give you medicine so you wont have either the mania or depression.
But thats just my experience..