Have you ever told someone they had something stuck in their teeth and they say something sheepish or defensive like, "I know. I wanted it there." ?
I've a coworker that said that... I think he was so afraid to make a low impression of himself that he didn't want to admit that spinach in his teeth was not a weakness for which any of us would've held it against him... of course his response made me think, "How pathetic." Heheh... :)
I was thinking... "Riiiiiiight. So now we're putting spinach in our teeth as a new trend in mouth decorations!"
When someone points out you've got a booger, thank them. I'd want to know if I had one. it's a simple matter of common courtesy, not their attempt to point our your character flaws... sheesh. :)
Examine my face in a mirror. (Update: get horrified on how bad my hair looks. Try to fix hair. Forget about the something on my cheek entirely.)
Exception is if I'm in the crazy building Z of the university which is ten stories high but doesn't have a mirror in any men's room, nor even traces to show that there ever was a mirror installed. In that case, I'd try to rub my cheek with a paper tissue.