All 2012 models either have the TPMS warning or display. The warning lights a little lamp when pressure is low. The display also shows you the tire pressures. The TPMS sensor is the base part of the valve stem.
A tire shop can use their scanner and determine if there are sensors installed and if they are working. All models except the SRT8 use the same sensors. I think I read somewhere that pressure is sampled at 10 or 20 minute intervals, not continuously, but I dunno. So a blow out may not register unless you drive on it for while.
2011 Grand Cherokee Overland V8, 2009 Liberty Rocky Mt V6, 2000 Grand Cherokee Laredo I6, 1979 CJ7 I6 Quadratrac