Rat:
There are two commercial items that are made to monitor radon systems. One is a 'negative pressure' guage that is installed in-line in the 4" PVC pipe, somewhere where a HO can 'look' at it to see if the fan is operating. This install is prevelant at an apt complex for the 'grade' units; guage is installed outside, for mgmnt and tenant to see.
The other is an 'alarm' that sounds if the fan is not functioning. These devices are prevelent in town house complexes, and the sensor device is in-line. This is the method of choice in a newer townhouse complex. The condo units have the radon units tied into the central station FA system for monitoring.
You should have some 'exposed' radon 4"PVC that you can mount the sensor into; the 'alarm' type gets power from a wall wart. You should use a device that is UL listed/labeled, I suggest contacting the radon fan mfg.
