When I went for my initial memory/mental health checks the person who tested me said she could tell I had ADHD even without the tests because I kept changing the subject and moving on to something else. Think of a Ronnie Corbett 'Two Ronnies' story on steroids and you'll get the idea. The difference is that Ronnie usually ended up where he started.
There are lots of check points. Quick-wittedness, flashes of genius, drinking too much, spending too much, the constant search for something else to make me feel good, unable to stay in a job for too long - either by getting the sack or suddenly blowing up and handing in my notice (or just walking out and not returning), impatience, risk-taking with no thought of the outcome or how it might affect anyone else, and generally a Jeykll and Hyde personality.