1.    When required to remain seated, a person has difficulty doing so.
2.    Stimuli extraneous tot he task at hand are easily distracting.
3.    Holding attention to a single task or play activity is difficult.
4.    Frequently will hop from one activity ot another, without completing the first.
5.    Fidgets or squirms (or feels restless mentally)
6.    Doesn't want to, or can't wait for his or her turn when involved in group activities.
7.    Before a question is completely asked, will often interrupt the questioner with an answer.
8.     Has problems with job or chore follow-through, and this difficulty doesn't stem from some other learning disability or defiant behavior.
9.    Can't play quietly without difficulty.
10.    Impulsively jumps into physically dangerous activities without weighing the consequences.
11.    Easily loses things such as pencils, tools, papers, etc.
12.    Interrupts others inappropriately, butting in when not invited.
13.    Talks impulsively or excessively.
14.    Others report that the person doesn't seem to be listening when spoken to.

There is some attempt to classify ADD more properly, I believe, as a neurological condition, rather than a psychological one. The main distinguishing characteristic of "ADD" brains is that they have very different patterns of blood flow and glucose utilization than "average" brains. This could become the basis for a more accurate and physically based diagnostic criteria, but I haven't heard of anything of the like existing.

Thom Hartmann goes on (on page 10 and 11 of Focus Your Energy) to quote from the work of Edward Hallowell and John Ratey, Driven to Distraction, in which they define criteria specific to adults. They define an adult as having ADD if he or she meets 12 of the following characteristics, with the caveat that a positive diagnosis is based as much on the severity of symptoms as a yes or no answer to the questions:

1.    A sense of underachievement, of not meeting one's goals (regardless of how much one has actual accomplished).
2.    Difficulty in getting organized.
3.    Chronic procrastination of trouble beginning a task.
4.    Many projects going on simultaneously; trouble with follow-through.
5.    Tendency to say what comes to mind without necessarily considering the timing or appropriateness of the remark.
6.    A restive search for high stimulation.
7.    A tendency to be easily bored.
8.    Easy distractibility, trouble focusing attention; tendency to tune out or drift away in the middle of a page or a conversation, often coupled with an ability to hyperfocus at times.
9.    Often creative, intuitive, highly intelligent.
10.    Trouble in going thorough established channels, following proper procedure.
11.    Impatient; low tolerance for frustration.
12.    Impulsive, either verbally or in action, as in impulsive spending of money, changing plans, enacting new schemes or career plans.
13.    Tendency to worry needlessly, endlessly; tendency to scan the horizon looking for something to worry about, alternating with inattention to or disregard for actual dangers.
14.    Sense of impending doom, insecurity, alternating with high-risk-taking.
15.    Mood swings, depression, especially when disengaged from a person or project.
16.    Restlessness.
17.     Tendency toward addictive behavior.
18.    Chronic problems with self-esteem.
19.    Inaccurate self-observation (both pro and con).
20.    Family history of ADD or manic-depressive illness or depression or substance abuse or other disorders of impulse control or mood.