code: def btc_tx_is_segwit( tx_serialized ):
      
      marker_offset = 4       # 5th byte is the marker byte
      flag_offset = 5         # 6th byte is the flag byte
      
      marker_byte_string = tx_serialized[2*marker_offset:2*(marker_offset+1)]
      flag_byte_string = tx_serialized[2*flag_offset:2*(flag_offset+1)]
  
      if marker_byte_string == '00' and flag_byte_string != '00':
          # segwit (per BIP144)
          return True
      else:
          return False
docs: Is this serialized (hex-encoded) transaction a segwit transaction?