  Different operators have been suggested in the literature to describe the
electron's spin degree of freedom within the relativistic Dirac theory. We
compare concrete predictions of the various proposed relativistic spin
operators in different physical situations. In particular, we investigate the
so-called Pauli, Foldy-Wouthuysen, Czachor, Frenkel, Chakrabarti, Pryce, and
Fradkin-Good spin operators. We demonstrate that when a quantum system
interacts with electromagnetic potentials the various spin operators predict
different expectation values. This is explicitly illustrated for the scattering
dynamics at a potential step and in a standing laser field and also for energy
eigenstates of hydrogenic ions. Therefore, one may distinguish between the
proposed relativistic spin operators experimentally.
