This is electrically a little different, but where I work we have these little [u]RS-232 Testers[/u] with two LEDs per line. We can see the LEDs flicker when data is transmitted and we can see the handshaking status, but mostly we use them just to make sure the RX & TX lines are connected correctly, or if we need to use a null-modem adapter. (If everything is connected correctly the green LEDs light-up for TX & RX, before any data is transmitted.)