Comment by LeonM
The one thing that is often a dead giveaway is how many stimulants seem to have the opposite effect on people with ADHD.
I have ADHD, amfetamines help me relax, caffeine causes me to fall a sleep, some anti-allergy medication can cause me to stay awake for 2 days straight.
I read that in some countries doctors can prescribe mild sleeping pills for babies to help them stay calm during long flights. They always advice to test it before going on the flight, because some babies can actually become hyperactive from that medication. If that happens, there's a good chance the baby has ADHD.
I think it still varies a lot between individuals. Caffeine often makes me more anxious, the effect on mental energy levels seems kind of random (either short burst of energy or just more tired mentally). Yet when I tried methylphenidate it made my mind calmer, clearly reduced anxiety and helped focus while increasing energy. At the same time it gave me pretty bad insomnia, stomach issues etc.
The best I can describe it is that I felt calmer in my mind, but overstimulated in my physical nervous system.