There is no foolproof way of differentiating between some of the encodings. While UTF-16 or UTF-8 with a BOM (such files don't necessarily start with a BOM by the way) "stand out" as being unlikely to be in any other encoding others are more troublesome.
Given any sizeable chunk of text, it ought to be possible to estimate the statistical likelihood of its being in a certain encoding/[language] even if it's in an unspecified 8859-* encoding. It would be quite an interesting exercise, but I'd be surprised if someone hasn't done it before. Perhaps someone here knows.
JD
