I can tell when DD is medicated by just looking at her, usually. It makes a big difference with her. She calms down a lot and acts a lot more mature. I can tell her to do a job and she does it, but when she's not medicated she is all over the place and even if I yell it doesn't necessarily get through to her. It also speeds up her processing. She recalls things like math facts a lot more easily when she is medicated.
We first had concerns when she was about 4-5 years. Before that she seemed very similar to other kids. Then I noticed she wasn't sitting still in circle time and the other kids were. She was the last to put her coats, boots, etc. on at preschool. We finally got the diagnosis when she was 6 and started medication right away. Some of the meds we tried had unacceptable side effects. It takes experimentation.