I would suggest thinking back over your childhood, and the way you related to the world around you, stress, boredom etc. And talking to people who knew you as a child about any distinctive (or “annoying” or “rude”behaviours.) Take note of things you/they remember.
I was dxd ADHD in my 60s, which made questions about my childhood hard, given that my parents died years ago & my sister & a close friend are both younger than me.
Be aware that “impulsive” doesn’t just refer to wildly rash actions & decisions. It includes interrupting people to finish off their sentences & impatience with people who take too long to get to the point, or play their turn in a game, drivers who take forever to park their car etc.