Optical detection of terahertz-frequency Rydberg transitions of neutral donors in GaAs is demonstrated via state-dependent resonance fluorescence from the neutral donor bound exciton (DX0) transition. The intensity of the elastically scattered light is proportional to the fraction of donor-bound electrons in the (bright) ground state; excited state and ionized donors are dark. Resonance fluorescence recovery after terahertz excitation yields a single-shot, time-resolved measurement of the lifetime of the lowest excited (2P−) state. We also demonstrate contactless detection of electron ionization/capture via the ionized donor exciton (DX+) transition.