I don’t notice which hand they use, but more often than not, lefties will write with their hand curved over (and smudging) the text they just wrote instead of from beneath (and not smudging.) That’s when I notice. So when I notice, I don’t say, “You know you could prevent getting graphite (or ink) everywhere…” I say, “Ah. Leftie.”
I have seen righties do the same weird smudging with the arced wrist. Why do people write like that?