lack of sleep and risky decisions even smaller ones! those are mine, a big one is, not eating but still having energy.

Along with what flore said, I’ve noticed that impulsive purchases or other impulsive actions I wouldn’t normally choose to do that way (or at all) are usually signs of an oncoming manic episode. I also have ADHD, though, so sometimes it isn’t mania. I’d say it’s a matter of examining how you are when not manic, when you’re under general stress, etc. Then compare to your manic episodes.