I hope this helps:

I wasn't diagnosed as bipolar until August 2015. In the past, I was diagnosed as having ADHD and MDD, so I didn't know what a hypomanic episode was or when one was occurring. Once I was diagnosed as bipolar, I started becoming more aware of my emotions, and I worked with my therapist to identify when I was experiencing hypomania. My therapist even said that it's difficult for many people to identify because everyone experiences hypomania differently. (The textbook definition is only that: a textbook definition. Plenty of people fit in the "NOS" category, which doesn't fit the textbook definition, so please consider that.)

It helps to keep track of your emotions, either mentally or on paper. Just write journal entries about your day and you'll start to see patterns. It can be a little boring, but when you think you're experiencing hypomania, you can look back at your previous entries and compare.