Quintessence and the Separation of CMB Peaks
  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. 
Michael Doran, Matthew Lilley, Jan Schwindt and Christof Wetterich