Shake-up transitions involving QW hole subbands have been observed as satellites in selective photoluminescence spectra of undoped (\mathrm{GaAs} / \mathrm{AlGaAs}) QWs. These shake-up transitions are explained in terms of an interaction between localized exciton and valenceband hole states attached to the QW subbands, in which holes are shaken up from the (n=1) heavy hole subband to higher subbands, either the (n=1) light hole subband or the (n=2) heavy hole subband. The required localization is due to the interface roughness; thus these new transitions are of intrinsic origin. From the observation of the intersubband shake-up processes we derive direct information about the hole inter-subband energies. Furthermore, the satellite intensity is strikingly enhanced in the presence of a magnetic field due to an increasing exciton localization related to the compression of its wave function in the field. The exciton wave function compression continues until its radius in the plane of the well is comparable with the radius of the "flat island" characterized by constant (\mathrm{QW}) width. Accordingly, from the magnetic field dependence of the shake-up satellite intensity we can roughly estimate the size of the "flat islands" and consequently probe the interface roughness.