I'd check for the use of multiple encodings in the same word (or sentence). That would be a dead ringer for this kind of thing.
Otherwise, something like this could help you - unfortunately it's only a partial table, and you'd have to use it in reverse.
On second thought, you'd probably have to add a list of "ignore-me" characters that can be added to a string to make it different while looking similar, for example U+0082. And now that I think about it, this could be used to defeat the "at most two encodings in each sentence". A word such as "déja vu" can be used legitimately (I remember seeing it out of a Mac editor), but the combining
U+0300 accent can be used to make "Víágŕa" look like something else altogether.
So first all "combinings" should be removed, then some legitimate characters must be ignored (e.g. the ellipsis - Word processors adore it... and the various styles of quotes). Finally encodings can be counted, or you can replace characters with their OCR lookalikes as above.