We present an experiment that probes polariton quantum correlations by exploiting quantum complementarity. Specifically, we find that polaritons in two distinct idler modes interfere if and only if they share the same signal mode so that "which-way" information cannot be gathered. The experimental results prove the existence of polariton pair correlations that store the which-way information. This interpretation is confirmed by a theoretical analysis of the measured interference visibility in terms of quantum Langevin equations.