A while back on OGR someone reported they can "hear" a TIU sending out DCS packets using an AM radio (not FM, only AM). Does anyone even have a pocket AM radio anymore? 
Additionally, there is this DCS Port Tester tool. The info is embedded in a long project thread in the Electrical Reference section.
Both of these are meant to indicate DCS "activity". It doesn't tell you if the high-frequency DCS signals are correct or mean anything...just that there is something that appears to be more than the AC track voltage. And something you can't detect/measure with a meter.
If you have a known good TIU you might be able to do a side-by-side and confirm that indeed nothing is coming out of any DCS channels. And even without sending TIU commands, when you first turn on a DCS channel you should get DCS Watchdog activity which should have an activity "pattern" that can be recognized.
