MULE: how to detect characters which can't be saved correctly.

[Jimmy Lao]

Somtimes when I want to save a file containing multi-byte charaters,
the following message appears:

---------------------------------------------------------
These default coding systems were tried to encode text
in the buffer `temp':
 chinese-gd2312
However, each of them encountered these problematic characters:
 chinese-gb2312: <some numbers listed here>
The first problematic character is at point in the displayed buffer,
and C-u C-x = will give information about it.

Select one of the following safe coding systems, or edit the buffer:
 utf-8 utf-16 utf-16 utf-16 utf-16be utf-16le iso-2022-7bit
Or specify any other coding system
at the risk of losing the problematic characters.
---------------------------------------------------------

Even if I choose utf-8, the buffer won't be saved correctly, as Emacs
doesn't really recognize these characters.

So, can I detect such problematic characters after I open the file, and
before I make any modification to the buffer? Which elisp function
should I use?

Thanks.

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