You can identify it a couple of ways:

• Mirror test: hold the filter backwards and look through it into a mirror. If the reflected image goes black, it’s a circular polarizer. If it stays clear, it’s a linear polarizer.
• Second polarizer test: stack it with another polarizer and rotate one. A linear polarizer passes polarized light, so at one angle the pair should go very dark. A circular polarizer has an added quarter-wave plate that “scrambles” the polarization after filtering, so it won’t black out in the same way.

Practical difference: a circular polarizer is basically a linear polarizer plus that extra quarter-wave plate. The main reason for circular polarizers is compatibility with many cameras’ autofocus and metering systems, which can be affected by linear polarizers.

Either type can reduce reflections and deepen skies or improve shots of water and glass, but if you’re using a modern autofocus camera, circular is usually the safer choice.