Enca is a (normally *nix) application which attempts to detect the text encoding of files.
It’s only supplied as source which isn’t too helpful for windows users.
Enca is particularly useful at guessing the encoding of text files where the language is known.
Here is version 1.12 compiled into a native windows binary: enca_1-12.zip
(I’ve compiled this from a downloaded source, so no responsibility is taken for it’s suitability for use, being free from viruses, etc. In other words, use it at your own risk)