Many detectors have been switched to "talk on defect only" to reduce radio traffic on the road channel. There are still a lot of normal ones out there though.
How does one distinguish between absence of defect and failure of detector?
I haven't heard that the RR's are setting them to talk only on defect, and frankly I'm not sure that's a wise thing to do. If the detector doesn't talk at all, then one doesn't know that it's actually working.
Now, I do know that many RR's have been shortening the broadcast by the detectors. Many detectors used to give you all sorts of neat info, like speed, axels, temp, and train length. Now many just say, "no defects" and give the MP number. Additionally many detectors orginally would broadcast an initial message when the engine passed over it to let you know that it was working, and then would broadcast a second time after the train had fully passed to give the report. Most RR's are working to eliminate that pre-broadcast, as the both engineer and the conductor should know when to expect a detector.