[I18n-sig] Given a string, how do you know the encoding/charset ?

[chas]

Hello to all,

Please forgive my ignorance but I haven't been able to find
an answer to what I thought was going to be a basic question:
Given a string of unknown origin, how does one calculate
or find its character-set ?  Is there any such module or
is there any way to use the existing codecs for this ? 

Is the only way to recreate something like
http://www.mandarintools.com/codeguess.html ?
And is there really so much need for 'guesswork' ?

Thank you very much for any pointers,

chas