We study the mechanism of single top quark production at the CERN LHC in the framework of an effective electroweak Lagrangian, analyzing the sensitivity of different observables to the magnitude of the effective couplings that parametrize new physics beyond the standard model. The observables relevant to the distinction between left and right effective couplings involve in practice the measurement of the spin of the top quark and this can be achieved only indirectly by measuring the angular distribution of its decay products. We show that the presence of effective right-handed couplings implies that the top quark is not in a pure spin state. A unique spin basis is singled out which allows one to connect the top decay products angular distribution with the polarized top differential cross section. We present a complete analytical expression of the differential polarized cross section of the relevant perturbative subprocess including general effective couplings. The mass of the bottom quark, which actually turns out to be more relevant than naively expected, is retained. Finally we analyze different aspects of the total cross section relevant to the measurement of new physics through the effective couplings. The above analysis also applies to top antiquark production in a straightforward way.