To add onto this there’s also lengthy cognitive testing (usually about 6-8 hours of tests) that is much more precise. My understanding from talking to my psychiatrist is that there are certain traits, such as reaction speed, that are correlated extremely strongly with ADHD. Usually patients with ADHD will test averagely on most of the cognitive tests and then perform several standard deviations below average on exactly those tests.

They don't only ask you questions, but will also ask your friends or parents for a second opinion on what you're like or how you were as a child. This happens more often if you are a child, of course.