Set voltage to 2V / Div and use a probe on X10 mode ( RS232 can go +12 -12 in the worst cade scenario ) Just connect one probe on RX and GND, another probe on TX and GND, adjust the time base according to the baud rate , supposing some communication is happening between your devices, you will see one signal for tx and another for RX. If you have the decoder option enabled, you can even see in data format