UTF-8 is a bit more complex than that.

The first byte in a sequence basically tells you how many bytes are following by looking at the top-most bits.

    0xxxxxxx => no followup bytes, just the 7-bit ASCII code
    110xxxxx => 1 byte follows
    1110xxxx => 2 bytes follow
    11110xxx => 3 bytes follow

The followup bytes then always have the pattern 10xxxxxx.

E.g. the only bytes that have bit 7 cleared are the single-byte 7-bit ASCII character codes.