Want to know the polarity of your piezo ceramic element? You could bang on it, and infer the polarity from the voltage generated.

An easier way to do this is to apply heat slightly and note the positive voltage. Even your breath will do as a heat source.

The terminal with the higher potential deserves a "dot" signifying that a positive voltage was applied to that terminal during poling.

Heat causes change in spontaneous polarization, which then creates surface charges due to charge conservation.