  Ramsey interferometry provides a natural way to determine the coherence time
of most qubit systems. Recent experiments on quantum dots however, demonstrated
that dynamical nuclear spin polarization can strongly influence the measurement
process, making it difficult to extract the $T_2^*$ coherence time using
optical Ramsey pulses. Here, we demonstrate an alternative method for spin
coherence measurement that is based on first-order coherence of photons
generated in spin-flip Raman scattering. We show that if a quantum emitter is
driven by a weak monochromatic laser, Raman coherence is determined exclusively
by spin coherence, allowing for a direct determination of spin $T_2^*$ time.
When combined with coherence measurements on Rayleigh scattered photons, our
technique enables us to identify coherent and incoherent contributions to
resonance fluorescence, and to minimize the latter. We verify the validity of
our technique by comparing our results to those determined from Ramsey
interferometry for electron and heavy-hole spins.
