I jumped over from the bipolar forum when I saw this. The two are so much alike. I've read the deciding factor between the two are the mood swings, or lack thereof.
If it's something that developed as a child, it's a little easier. Kids with ADD frequently get in trouble in school but they usually don't mean to start anything. Usually. Bipolar kids have moods where they sort of go looking for trouble. They like to challenge authority, at least that's what I've read. I know it was true for me as a teenager.