Dilated pupils. Tense jaw. Muscles around the eyes are slightly more tense (small wrinkles temporarily disappear for example). People react faster and speak more coherently (or if they took too much, incoherently but very fast). Dry mouth (very slight lisping or tongue sticking when they talk). Straighter back, clenched fists, grasping or fidgeting hands that they don't normally have. A general appearance of alertness. Unusually impressive performance in a social situation.

Some combination of the above, but it's pretty obvious once you get used to it. At the same time, I'm pretty sure many people in my life have no idea that I take stimulants (though as I said, I don't hide it), but just see me as being at my best when I'm on them.

Flinches if surprised, picks at skin or scratches more than normal (there might be visible sores from this), intense preoccupation with things ordinarily beneath interest, hard to interrupt from work, drinks lots of water if responsible, particularly unconcerned with thirst if not. Particularly prone to discuss subjects at length, perhaps an unusually quiet or loud vocal tone.

There's also of course the side effect list, which usually reflects this sort of thing: https://www.drugs.com/sfx/amphetamine-side-effects.html

(Excess) scratching, excess fidgeting. Staring off into the distance in the same direction for a while. Overconfidence and they really like to talk about stimulants ;-).