First of all: you can't have a "Unicode" file.

You can have a file containing Unicode code-points encoded in one of the transformation formats defined by the Unicode standard, such as UTF-8 or UTF-16.

So the question becomes:

I have a byte-stream. Is it a valid (ISO-8859-1|UTF-8|UTF-16)-encoded representation of some text?

This can be answered, since none of those encodings defines a meaning for each and every byte-sequence. But this is quite possibly not the answer you're looking for.

The way I see it, it's easier to check if your byte-stream contains something you know not to be text, using something like file(2) or File::MMagic as already suggested.

Doing it the other way ("is it a valid encoded form") gives you a lot of "this is text" when, in fact, it is nothing intelligible.

You could try to decode it and then do some heuristics to see if looks like text (ex. a lot of letters from the same script/writing system in a row, or something of the sort), but I think it's more trouble than it's worth.

-- 
        dakkar - Mobilis in mobile