What is the difference between zero and multiple quarter wave plates?

[abbas_majidi]

Hello
I have a quarter wave plate for 550 nm but i can't determine which it is a zero quarter wave plate or multiple.

How can I find m in this formula :
delta phase =m*2 Pi +Pi/2
?

Thank you.

 

[PietKuip]

You can test with other wavelengths, assuming that its indices of refraction (and their difference) does not have too much dispersion.

 

[abbas_majidi]

You can test with other wavelengths, assuming that its indices of refraction (and their difference) does not have too much dispersion.

But I don't know indices of refractive. Actually absolute phase difference is not specified. I know relative phase difference which is equal to Pi/2 only.