THC oil doesn't mix with glycerine. You can mix it with propylene glycol if you are using other diluents as well.

They decrease viscosity, making it obvious that a product was diluted.

Scientific wild-ass guess: Polyols (ethylene glycol, propylene glycol, glycerine) are very hydrophilic. It would be trivial to detect presence in oil concentrates - just shake with a known quantity of distilled water. The polyols will be pulled into the water layer and increase the volume. It's not quantitative but if you know the 'ol in question you can calibrate a curve and get within a few percent of actual with just graduated cylinders.