We consider how Higgs Inflation can be observationally distinguished from variants based on gauge singlet scalar extensions of the Standard Model, in particular where the inflaton is a non-minimally coupled gauge singlet scalar (S-inflation). We show that radiative corrections generally cause the spectral index n to decrease relative to the classical value as the Higgs mass is increased if the Higgs boson is the inflaton, whereas n increases with increasing Higgs mass if the inflaton is a gauge singlet scalar. The accuracy to which n can be calculated in these models depends on how precisely the reheating temperature can be determined. The number of Einstein frame e-foldings N is similar in both models, with N = 58-61 for singlet inflation compared with N = 57-60 for Higgs inflation. This allows the spectral index to be calculated to an accuracy \Delta n = 0.001. Provided the Higgs mass is above ~ 135 GeV, a combination of a Higgs mass measurement and a precise determination of n will enable Higgs Inflation and S-inflation to be distinguished.