ADD/ADHD symptoms may initially appear to be the same as a child just being a normal child. However, it takes a trained professional to distinguish the two. ADD/ADHD are overdiagnosed mostly because the people responsible for diagnosing the situation often don't have the time to study the child properly. The biggest clue is when they are given stuff that would normally make a child hyper, instead they get calm. That is the best indicator. This won't happen once or twice, but become the way that the parent deals with their hyper behavior.
Usually children with ADD or ADHD are much more extreme in their behavior then that of other young children. While typically children may have periods of hyperactivity and inattentive behavior, these behaviors in a child with the disorder are often very extreme and never ending. Also children with the disorder can easily be distinguished because they are typically suffering in school and social interactions.
I do not think that ADD/ADHD can be distinguished from the normal behavior of children. I think that the disorder is something that was made up for the sake of medical drug companies profiting from the treatment for it. I think too many kids are being diagnosed of having ADD/ADHD.
Naturally there are extreme cases as there are with any diagnosis where you can clearly see that someone has the problem, but in many cases we just do not want to deal with the restless behavior of young boys and their different learning styles and would rather medicate them than deal with them.