Its the energy level for me. I can tell what it is by how my body feels. I do not get the energy fluctuations in normal life ups and downs.

Thought patterns changing is another one. I journal a lot so i can see when the thoughts start to change.

Paranoia, OCD and anxiety sometimes warn me that a mood change is coming.

I do not rapid cycle, so my bipolar moods need to last at least 2 weeks, if they do not last 2 weeks they do not constitute a bipolar mood. If they are triggered by anything I also do not count them as BP unless it spirals into months long episodes.