(Auto-)detecting which XML encoding is necessary/requi

Sometimes the situation occurs that an XML document does not fit to its own encoding setting in the first line.

Is there a way to find out the best/necessary/required encoding setting for an XML doc resp. what
I have to write in the first line:

<?xml version="1.0" encoding="iso-8859-1"?>

or

<?xml version="1.0" encoding="UTF-8"?>

or simply

<?xml version="1.0"?>

Ben