Assuming you want to see what is arriving at the Pi's serial port.
The Pi's primary UART is connected to pins 8 (TXD, gpio 14) and 10 (RXD, gpio 15).
The grounds of the two machines needs to be connected as well as TX<--->RX and RX<--->TX.
All the Pi's gpios are 3V3. If you feed a 3V3 TTL (i.e. 0V or 3V3) to gpio 15 you can see what is being received with the following pi commands.
stty -F /dev/ttyAMA0 9600 # or the baud rate you are using
Press ctrl-c to stop. If the data you are sending is not text try the following instead
cat /dev/ttyAMA0 | od -x
The UART is normally used for boot messages and as a login console.
You could log in to the Pi via the serial link and a terminal emulator running on a PC.
In that case the PC's baud rate needs to be set to 115200 8N1.
Perhaps try that first.