Generally some of the more common symptoms of ADD tend to effect you attention span. one good indicator, is if when your reading a rather mundane, yet very informative book on a subject you very interested in, your mind might start wandering. You'll be reading the words in the book, but you'll be thinking about something else. And after a couple pages you'll realize you have no idea what you just read

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This isn't to be confused with your mind wanders when reading a book you despise, on a subject you hate. This is normal in alot of people :P
Aside from that, just a general inability to concentrate on redunant or mundane tasks, or a mental block after a sustained period of lecture is a good indictor of ADD.
P.S.
It has also been theorized that The format of TV programming has caused shortened attention spans in the TV viewing public. Because the general network TV stations generally play shows in 8 minute increments, people who view large amounts of TV programs, especially in early years when the brain is devloping most, are conditioned to have shorter attention spans compared to people who indulge in books or commerial free movies. So i suggest talking abd reading to your children rather than sitting them infront of a TV.