Both of the magnetic and piezo buzzer are black cylinder with two pins by appearance. There are a few methods can help you to determine the buzzer you have.

1. Testing the buzzer by a magnet or another same buzzer, if attracting, which means the buzzer you have is magnetic buzzer. On the contrary, piezo buzzer has copper plate inside, attraction does not happen.

2. Measuring the outer diameter of the buzzer, the common outer diameters of magnetic buzzer are 6.7mm, 9.0mm, 9.6mm, 12.0mm, 14.0mm, 16.0mm, and 25.0mm. Besides these diameters, others are all piezo buzzers.

3. Looking inside the buzzer sound hole, if you can find a yellow inner copper plate in a large size buzzer, it must be a piezo buzzer. However if you find a white iron color plate, then you cannot say it is a magnetic buzzer, because some piezo buzzers are also made with alloy plates.

Introduction of magnetic buzzer - Introduction of piezo buzzer