I understand your concern -- a package method might be called by one process handling utf8 data, or by another that's handling something else. (Should we be forced to make sure the latter is revised to conform? That depends...)

The Encode man page in the 5.8.0 distribution seems to have an answer, but maybe not an ideal one. Check the sections titled "The UTF-8 Flag" and (within that) "Messing with Perl's Internals"; if you can trust Perl's application of the UTF-8 flag on scalars in your application, then you can test this flag. (But maybe the nature of the test will change in a later release.)