Easiest way to detect is to use the LM15A/lamp that is visible from the outside (or purchase a X10 light switch, appliance module to the do the same) to sense the ARM state thru the power line. During ARM the SC1200 will flash the house/unit code you programmed into your unit. The sc1200 only gives positive feedback during ARM and PANIC/Alarm Trigger through the power lines... no RF signalling
Unfortunately from your hardware listing, you don't have the hardware to "read" the power line signals. The CM15A has this capability, however this piece of hardware is out stock with its replacement in TBD state.
Let us suppose that you you had the proper hardware, I have only seen solutions to detecting PANIC or Alarm Trigger state only. Maybe there's some one out there that can chime in solving this problem using off shelf X10 software and hardware.
My home grown windows service application is niche to my particular situation (sends email on ARM, PANIC/Trigger), but I never bothered making it general purpose for public consumption months ago when I saw both the SC1200 and CM15A go out of stock.