On my other Jeeps where I used it the engine is warm and when you first start it you should have almost instant warm air coming out of the vents. At times you could even tell on the hood if wasn't too cold that the snow would slowly melt.
Other than that you can't really tell that it is working.
2006 Black Grand Cherokee SRT8, Magnaflow, 4" BWoody + K&N, Remote Start, BT Catch Can, Tinted Tails, Rear fogs,
2006 Mercedes CLK500
2004 Silver Grand Cherokee Limited
1997 Black Trans Am Ram Air, Borla