  We propose that it should be possible to use the CMB to discriminate between
dark energy models with different equations of state, including distinguishing
a cosmological constant from many models of quintessence. The separation of
peaks in the CMB anisotropies can be parametrised by three quantities: the
amount of quintessence today, the amount at last scattering, and the averaged
equation of state of quintessence. In particular, we show that the CMB peaks
can be used to measure the amount of dark energy present before last
scattering.
