Level of energy vs sleep is my biggest marker that I'll pick up on.

Lots of sleep and still exhausted: probably in some level of depression.
Very little sleep and full of energy: hello hypomania!

I also (sometimes) notice my mood changes. Depressions are pretty obvious - things that normally would make me happy or excited just don't matter or make a difference. Hypomanias are rather trickier - I might find myself NOT being upset/caring about the things that would normally upset me. I'll be able to just brush them off and keep up the good mood!

I also have large social changes - In a depression I will withdraw and want to spend a lot of time by myself because everyone else will be stressing me out. When I'm hypomanic, I want to go out ALL THE TIME and will even want to be drinking on worknights (although I can resist that as the idea of working with a hangover is NOT my idea of fun!). People can't usually keep up with my pace when I'm hypomanic.