CHADD is by far the best organization with tons of local support. Web MD has good check lists.
I raised a child with extreme case of ADHD and bypolar.
The final diagnosis comes with the medication. If the child is not ADHD the medication will complete back fire. Also You can not over medicate these kids. Feel free to email me and I will talk with you over the phone.
In some states it is manditory that the child with ADHD gets treatment.
If you are the parent I would personally urge you to get help. 60% of mothers who raise these kids are clinically depressed and 85% end in divorce. They are very difficult to raise but awesome when they find their way. Most presidents of software companies have it.
This is the most researched disorder there is. There is alot you can do. But the sooner the better.
Again I urge you to get real advice from CHADD.