Situation 1: Equilibrium. Needless to say, the last pile is genuine.
Case 2: Lift or sink one end, and take out two parts from the tray for the second measurement. Put one aside.
Situation 1: If you take the parts out of the hoisted tray, then the parts in the hoisted tray are genuine!
Case 2: If the parts are taken out from the sinking balance, then the sinking tray is genuine.
I hope you understand.