Yet another method based on file(1) behavior:
>>> textchars = bytearray({7,8,9,10,12,13,27} | set(range(0x20, 0x100)) - {0x7f})
>>> is_binary_string = lambda bytes: bool(bytes.translate(None, textchars))
Example:
>>> is_binary_string(open('/usr/bin/python', 'rb').read(1024))
True
>>> is_binary_string(open('/usr/bin/dh_python3', 'rb').read(1024))
False