We investigate the bias-dependent photoluminescence (PL) response of a WSe₂/MoS₂ heterostructure diode. Under forward bias, trion-related emission in WSe₂ is markedly enhanced, whereas reverse bias produces minimal spectral changes. This asymmetric response cannot be explained by electric-field effects alone and instead reflects carrier injection at the functional interface. These results demonstrate that PL provides a direct optical probe of carrier injection in an operating van der Waals heterostructure. More broadly, this approach establishes spatially resolved PL spectroscopy as a versatile tool for pinpointing the interfaces governing device operation in van der Waals heterostructures.