How can I test the encoding of a text file... Is it valid, and what is it?

I have several .htm files which open in Gedit without any warning/error, but when I open these same files in Jedit, it warns me of invalid UTF-8 encoding... The HTML meta tag states "charset=ISO-8859-1". Jedit allows a *List of fallback encodings* and a *List of encoding auto-detectors* (currently "BOM XML-PI"), so my immediate problem has been resolved. But this got me thinking about: What if the meta data wasn't there? When the encoding information is just not available, is there a CLI program which can make a "best-guess" of which encodings may apply? And, although it is a slightly different issue; is there a CLI program which tests the validity of a *known* encoding?