how to find encoding violations in Emacs buffer?

[riccardo . murri]

Hello,

from time to time, a buffer gets some spurious character in and Emacs
refuses to save it in the correct encoding. So I am presented with the
choice of other different encodings.

However, in most of the cases, I know that the file *should* be UTF-8
encoded.  So I would rather like to find out where the offending
character is and correct it, instead of choosing a different encoding.

Is there any function/package/elisp hack to find/highlight characters
in a buffer that Emacs could not encode as UTF-8?

Thank you for any hint!

Riccardo

P.S. Currently running 22.0.90