If you want the diagnostic criteria, do a search on google for ADD/ADHD and DSM-IV.
Personally, i have it. I can't sit still for too long, and have hard time concentrating. If i'm talking to somebody face to face, i literally zone out after like 30 secs and have no idea what they're talking about... UNLESS the thigns i'm doing, person i'm talking to, etc are interesting. Then my concentration is complete and i actually concentrate/listen better than most people. That's why with the classes i like, i don't bother taking notes... even if i did, it's not like i would review them.
But lack of concentration isn't the only issue with ADD. It's inappropriate concentration. For instance, if i'm excited about something, such as working out, i'll completely devote all my time and energy to it. I will spend hours upon hours reading about it, looking through the websites, and even spending hundreds of dollars on gloves, straps, belts, mp3 players, etc. My interest in it causes me to neglect everything else. But this interest doesn't last long, sometimes days, sometimes weeks, and if i'm lucky, months. But when my interest wanes, i can feel it. I KNOW i'm losing interest, and i KNOW what's going to happen next; i'm going to lose all interest, all the time and money i've spent in it are going to be a waste, and i'm going to get obsessed with something else. It's like being high, and feeling great, but at the same time knowing that it's going to come crashing down around you.
The reason why it's difficult to assess children properly for ADD is because children are not as cognitive or expressive as adults are... so they can't express what they're going through. All you can go by are their observed behaviors.
I wished i was properly diagnosed when i was little... i was placed in a remedial class because of it. But a year later in the remedial class, i was placed back in my normal grade. A year in my normal grade, and i skipped a grade.
The first time i read up on ADD, it was probably one of the most profound experiences i had. Finally something that made sense to me, why i act the way i do, why i get so obsessed and excited over things, why i zone out when i talk to people face to face, etc.