Take a wire from the RS232 TX output and connect it to the RS232 TX input.
Then send a byte out, read a byte back and print it out so you can see it on the serial monitor. If everything is working and wired up correctly you should get the same byte back as you sent out.
If you get nothing then it is wired up wrong, broken or something else.
If you get back what you sent then your problem is with the connection between your device and the RS232.