How to find the spin of a particle in its own reference frame?

Hello experts,

I am trying to track the spin of a charged particle inside a complicated EM field.
Right now I have a UserSteppingAction where I get the polarization with:

CLHEP::Hep3Vector polarization = track->GetPolarization();

If I track polarization.getTheta() this seems to change as the direction of the momentum changes.
Is there a way to get the spin with reference to the momentum of the particle?

Kind regards,