Have a look at the encoding rules of UTF-8.

A valid UTF-8 sequence starts either with 0b0xxxxxxx or with 0b11xxxxxx. So any octet starting with 0xb10xxxxxx is invalid UTF-8:

> perl -wle "print sprintf '%08b', $_ for (0xa9,0xae)"
10101001
10101110
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An untested easy check could be to match your string against /[\x80-\xBF]/, which are the hex representations of the bit patterns we've identified:

perl -wle "print sprintf '%08b - %02x', $_,$_ for (0b10000000,0b101111
+11)"
10000000 - 80
10111111 - bf
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