Here is something I stumbled across at Vat10.com:

How To Address An Embarrassing Problem

Stefanie suggests that if it is above the neck and below the knee, then it could not be potentially embarrassing if you whisper it to them.  If it is anywhere else, taking a cautionary approach is adviced.

How do you deal with it when your co-worker has food on his face, or something stuck on his back?  To tell or not to?