Using MCF51JM128 Coldfire V1
Strip the encapsulation off the chip and use a very good microscope.
There's no way to program the chip to route an internal signal to a pin.
Software has to poll the status register and then drive a GPIO pin for you to monitor.
Note that IDLE sets on IDLE and clears when the software clears it. it is not "live". Any polling you do might upset your "normal code" as it might interfere with the normal interrupt operation.
Are you trying to "prove" the idle timing? You should be able to work out after how many bits it should assert by looking at the serial port data.
Retrieving data ...