usually it's pretty obvious once we get the dose right
Most patients have no difficulty telling that the medication is working when the dose is right. In practice, any of the symptoms reported in the assessment, or even on a screening test will likely improve to some degree, often dramatically. It's not usually necessary to keep a diary or dither about whether it's real or placebo - when we get the dose right, life should be a lot easier. Some people expect the medication to change them but it doesn't other than they don't have the adhd symptoms they had previously. Other people (more severe ADHD, anxiety, depression) do feel fundamentally different on medication because the ADHD impact was huge and getting it under control is a giant relief.
In patients who will be in school but aren't currently, I have them test focus by doing some reading, or taking one online course and testing the waters or deliberately paying attention to how they're doing in terms of procrastination, listening, misplacing things and being impulsive.