We study theoretically the effect of spin-orbit coupling and sublattice asymmetry in graphene on the spin polarization of photoelectrons. We show that sublattice asymmetry in graphene not only opens a gap in the band structure, but in the case of finite spin-orbit interaction it also gives rise to an out-of-plane spin polarization of electrons close to the Dirac point of the Brillouin zone. This can be detected by measuring the spin polarization of photoelectrons, and therefore spin-resolved photoemission spectroscopy can reveal the presence of a band gap even if it is too small to be observed directly by angle-resolved photoemission spectroscopy because of the finite resolution of measurements or because the sample is p-doped. We present analytical and numerical calculations on the energy and linewidth dependence of photoelectron intensity distribution and spin polarization.