Theoretical studies of Manz et al. have shown that upon excitation of a Mg-porphyrin molecule with a circularly polarized laser pule, a ring current can be generated that is much stronger than what can be induced by means of an external magnetic field with present technology. We show that the circular dichroism signal of a Mg-porphyrin molecule that has been excited to a state with an inner ring current is proportional to the magnitude of the probability of this ring current and can be used for the detection of this current. In analogy to magnetic circular dichroism, it probes the symmetry of degenerate excited states.