My co-worker has it and he's got it REAL BAD...
Anyway, first and foremost, you don't develop it as an adult. The first clue thing they will try to figure out is if you have had it all your life. How were your grades in school, how did you do in school...that kind of stuff.
Then they will go into how you are functioning in your daily tasks.
I think Adult ADD is much easier to diagnose than ADD in children because you can verbalize your symptoms better. Unfortunately, most people who have ADD have a hard to recognizing the symptoms in themselves.
As I said, my coworker has it. Once in awhile he goes on a drug holiday and he drives us all up the wall, can't do his job, very disorganized, etc. The funny thing is, we can all see the change in him but he is totally unaware of it.