The Tether Manager service (tetherman) publishes tethering session information to the /pps/system/navigator/status/tethering object. When a tethering session has been started or stopped, this object is updated with the new tethering state (on or off). Here's a sample object:
@tethering
[n]dat:json:{"tethering":{"state":"off"}}
For a full description of the contents of this object, see the entry for /pps/system/navigator/status/tethering in the PPS Objects Reference.