Nina Sharp says the Pauli exclusion principle says that "no two objects can occupy the same space at the same time". This is an oversimplification in several ways. First, the principle applies to fermions (a class of subatomic particles ), not "objects". Second, the principle applies to quantum states, of which position ("space") is only one part.
Since the shapeshifters have so much mercury in their blood, a simpler method of detection (although not nearly as 'Fringe Worthy' ) would be to use a standard mercury sniffer. These are used for cleaning up mercury spills and can detect mercury well into the part per million range. One of these detectors would peg out near any shapeshifter.