Working serial connection between Arduino serial port and PC is demonstrated by flashing of both RX and TX LEDs during ANY program "Upload".
The physical RX path can also be verified by using and running Examples Basic BareMinimum and monitor.
Just type in random series of characters - using PC keyboard "repeat" feature and than pres "Send" or just key in "Enter".
Flashing "RX" LED indicates good RX connection and is a good indicator that TX works also.
Setting the baud rate to 300 bauds makes the RX flashing more pronounced during this simple verification test.
Software remote loopback test (monitor as a source and Arduino as a remote destination) can be implemented by echoing ( using Serial ) the characters typed into monitor "send" field back to the monitor "print" field.
Observing both LED's will verify the connection.
Software remote test "sketch" available upon request.