If you do not know what is the output from one port or the input, i.e. which is RxD or TxD, the oscilloscope is a good way to find out. The data pin coming out of a serial connector is generally going to show a high voltage greater than +/- 5 volts. The data pin going into a serial connector will generally show a low level a few tenths of a volt above or below ground when the input id open.
Michael Karas