I recall a thread a while back about someone observing you can hear DCS activity on an AM radio? Do they even sell pocket AM radios any more?!
Anyway, the idea might be to place the radio on the track section or loop in question and listen for activity. I don't know what it would sound like but I'd think you'd hear "bursts" of static? Obviously there are loose ends to resolve such as interference from other active DCS channels and what not...