I currently have a little boy (4 next month) that I am absolutely positive could be "diagnosed" as having ADHD. I can guarantee you that if I attempted to broach this subject with mom, she would brush it off. She does this currently with various behaviours already. Me: So and so was not making very good choices today, he blah blah blah". Mom: "Well, he is 3 you know!" Um, ok. The other 3 year olds seem to be able to control their impulse to throw food at their friends?
Anyway- they need to be open to hearing it first, as other posters have said. And some parents may never be willing. I think you can bring up the things you have noticed, but avoid using a label at all costs!