There's no 100% solution to your question, because even Unicode file may be considered as binary or even ascii.

AFAIK usually unicode file starts with "\xFE\xFF" or "\xFF\xFE" but this is not always true.

If a file is created by your own program, then, for example, use a special naming system, for example unicode text files let have an extension ".utxt" and ascii files just ".txt"

Courage, the Cowardly Dog.