Sure. Using byte-wise processing, all UTF-8 characters with character code >= 128 must match the following pattern:

  /[\xC0-\xFF][\x80-\xBF]+/
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(Actually you can even put more stringent constraints on the byte sequence, but this will do for a start.)

It means that if you encounter anything matching /[\x80-\xFF]/ outside what's matched by the above pattern, it's not (valid) UTF-8. You can do this, for example, by using this:

my($utf8, $bare) = (0, 0);
use bytes;
while(/(?=[\x80-\xFF])(?:[\xC0-\xFF][\x80-\xBF]+|(.))/g) {
    $bare++ if defined $1;
    $utf8++ unless defined $1;
}
print <<"END"
utf-8: $utf8
bare: $bare
END
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The idea behind the pattern is that the properly formed UTF-8 characters are eaten using the first alternative, and the remaining bytes by the second.

If $bare ends up with a value > 0, then it's not UTF-8. If the string doesn't contain any bytes with character code >= 128, then it doesn't matter which you choose. Both $bare and $utf8 will be zero, in that case.