ISOTP encodes the frame type in the first nibble of a frame.

def guess_payload_class(self, payload):
        """
        ISOTP encodes the frame type in the first nibble of a frame.
        """
        t = (orb(payload[0]) & 0xf0) >> 4
        if t == 0:
            return ISOTP_SF
        elif t == 1:
            return ISOTP_FF
        elif t == 2:
            return ISOTP_CF
        else:
            return ISOTP_FC