How to determine the polarity of a piezoelectric buzzer?
Normally when polarization is done a dot of paint or a scratch or some similar sign is written on one surface of piezoceramics to indicate positive polarity. To view the deformation probably the best system it shine a light ray on the surface and see how the reflected ray goes. Another system could be to mechanically couple a reference bimorph to unit under test and measure the output current from known unit when you give a drive to unit under test.