Partly it's learning your signs, and at least for me getting feedback from others (particularly my husband). Some of my low level hypomania signs are -
- finding the sky mesmerising in its beauty, when others think it's ordinary
- smiling so much my cheeks hurt, even when nothing good is happening, eg when driving to work
- dancing instead of walking
- not spending any long amounts of time just chilling out (watching tv, reading, generally sitting still etc)

I'm getting better at telling, but I still find the line blurry.