The theory of hole superconductivity predicts the existence of a charge imbalance in the quasiparticle excitations in superconductors, which gives rise to a thermoelectric effect of universal sign across tunnel junctions when one or both electrodes are superconducting. Recently, temperature gradients of appreciable magnitude have been experimentally achieved in NIS tunnel junctions, opening up the possibility of testing this prediction. However, a thermoelectric effect across a tunnel junction could also arise from energy dependence of the transmission probability across the barrier, independent of the nature of the superconducting state of the electrodes. We explore the consequences of these two effects acting simultaneously and to what extent it is possible to differentiate between them experimentally. It is concluded that careful experiments should be able to single out the intrinsic effect arising from the properties of the superconducting state. These experiments would yield a new measurable quantity that characterizes the superconducting state of a material, in addition to providing a stringent test of the theory of hole superconductivity.