How to identify the positive and negative poles of a button battery holder?

Typically, the side of a button battery marked with "+" is the positive pole, and the back is the negative pole; the shell at the edge of the button battery is also the positive pole. 

Therefore, the button batteries loaded in most button battery holders have their positive poles facing upward. This is because the side end of the button battery holder is equipped with a metal shrapnel that clamps the button battery to prevent it from falling out—and this is the positive electrode metal shrapnel, which fits closely against the positive edge of the button battery. Thanks to the large-area electrode contact of the positive electrode metal shrapnel, the stability of electrode contact is effectively improved.

 If you reverse the positive and negative poles of the button battery, it can still be clamped by the shrapnel, but the battery will short-circuit.