If the polarize filter is not marked you can easily test if its circular or linear by doing what’s call mirror test.
1. Hold the polarizer about mirror with filter threads pointing towards you. Look at your reflection in a mirror; you should see your reflection in the mirror (as if looking through a neutral density filter).
2. Now turn the filter around so that the filter threads point at the mirror.
If the polarizer is linear you should see the same thing that you saw before, but if it is circular it will appear black and you will not see the reflected image of you.