Ohm the L1 and ground terminal on the warmer. It should be infinite. Make sure to try it both off and on. And don't be afraid to rattle the unit around a little.
As for the miswiring, What the hell kind of electricians are they? That should never happen with a commercial electrical company. Ideal makes a simple plug in GFI tester that is inexpensive, and you should probably get one and test all the outlets in the building before acceptance. It has 3 LED's that will tell you of any miss wire. If they find one and correct it at the wrong box, it may make ones downstream wrong. It also has a button that adds a high resistance path to ground that should trip any GFI.
The wires are color coded as well as the terminals on the receptacles. Colors like red, black, yellow, blue, violet, and orange are always live. Orange is reserved for the wild leg in Delta systems. Neutral is always white or Gray in the US. Green is always ground in North America. European Machines can have a different color ground. And even colored neutral's.