I wondered the same thing years ago. In 2004, at age 59, I asked my primary care doc for a referral to be tested. He had me see a psychologist. I took a series of tests (audio). My wife and I filled out questionnaires, and I discussed my school history, work history, and living history (homes/apts./etc.). Beginni9ng at age 20, I had lived in 17 homes/apts. in 35 years. That does not include the 4 years I was in the military. Basically, I had moved every 2 years. Not including the military, I had 13 different jobs. When you consider one was being self-employed for 16 years, that means 12 of those jobs covered 18 years - about 18 months each. I was unable to be disciplined enough to do homework, so I dropped out of college after 2 miserable years. I was fortunate to graduate in the top 1/4 of my high school class without doing very much homework. I was diagnosed with ADD. Before I sought the referral, I read the book So You Think You May Have ADHD. I saw myself on about every other page.
If any of that rings a bell with you, find a similar book. I believe what I read is out of print; but, there are other self-help books on the topic that should help you. Good Luck.