Angular-dependent emission spectra are investigated in a strongly coupled InGaAs-GaAs-AlAs-based semiconductor microcavity as a function of excitation intensity and of detuning between the uncoupled exciton and photon modes. Under conditions of nonresonant excitation, it is shown that the onset of stimulated emission always occurs in the weak coupling regime. Angular-dependent studies show that the transition to weak coupling occurs when the linewidth of high k excitons becomes of the order of the normal mode splitting of the exciton-polariton coupled modes. We conclude that, under nonresonant excitation, "polariton lasing," where stimulated polariton scattering followed by photon emission occurs in the strong coupling regime, is only likely to be achieved in systems with larger exciton binding energy than in GaAs-based structures or possibly also in GaAs microcavities containing excess concentrations of free electrons. The experimental results below threshold are found to be in good agreement with numerical solution of Boltzmann kinetic equations for the photoexcited polariton distribution.