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Send an extend cell to relay A, listing the address and identity key of relay B but the wrong port.

Relay A calls circuit_extend() for the new cell, which calls channel_get_for_extend(), which tries to figure out if there's a canonical connection already established. To do that, it asks

    if (!channel_is_canonical(chan) &&
         channel_is_canonical_is_reliable(chan) &&
        !channel_matches_target_addr_for_extend(chan, target_addr)) {
      ++n_noncanonical;
      continue;
    }

and channel_matches_target_addr_for_extend() turns into channel_tls_matches_target_method() which basically is

  return tor_addr_eq(&(tlschan->conn->real_addr), target);

It doesn't consider the port. So if there is a canonical channel open, bingo we use it.

But if there isn't one open, then off we go to make one:

      n_chan = channel_connect_for_circuit(&ec.orport_ipv4.addr,
                                           ec.orport_ipv4.port,
                                           (const char*)ec.node_id);

where ec.orport_ipv4.port was set from extend_cell_parse(), i.e. it came from our extend cell. If we specify the wrong port, that connect attempt will fail. Now we can distinguish, remotely, which situation we're in.