In general I tend not to notice, especially minor things like a trim. And I have gotten in trouble for not noticing and remarking on a female who has had a trim.
I might notice more dramatic changes, but my reaction might be a bit out of the ordinary. There is a female I work with who had shoulder-length hair. One day she crossed my path in the halls at work; she looked at me, smiled, and waved; I looked at her with a blank, semi-confused look and really didn't acknowledge her, and she went around a corner. Just after she did that, I realized who she was, and that she'd had her hair bobbed down to about a third of its previous length.
What do I do? I stop, poke my head around the corner, and say, "Wait a minute." She stops and turns around, so I can see her face, and confirm it was who I thought it was. And then I say/ask, "You're missing some hair?" (duh), to which she laughs and responds "Yes." And I say, "Ah, OK." and duck back around the corner... Only then realized that I never actually acknowledged her greeting, poke my head back around the corner, and say, "Oh, hi!"
I have a feeling I did that completely backwards. And I blame it all on haircuts.
Good fortune,
- Icarus finds the placement of hair on humans to be bizarre...
_________________
Please forgive me if, in the heat of battle, I sometimes forget which side I'm on.