We introduce two new diagnostics of dark energy ( DE ) . The first , Om , is a combination of the Hubble parameter and the cosmological redshift and provides a null test of dark energy being a cosmological constant \Lambda . Namely , if the value of Om ( z ) is the same at different redshifts , then DE \equiv \Lambda , exactly . The slope of Om ( z ) can differentiate between different models of dark energy even if the value of the matter density is not accurately known . For DE with an unevolving equation of state , a positive slope of Om ( z ) is suggestive of Phantom ( w < -1 ) while a negative slope indicates Quintessence ( w > -1 ) . The second diagnostic – acceleration probe \bar { q } – is the mean value of the deceleration parameter over a small redshift range . It can be used to determine the cosmological redshift at which the universe began to accelerate , again without reference to the current value of the matter density . We apply the Om and \bar { q } diagnostics to the Union data set of type Ia supernovae combined with recent data from the cosmic microwave background ( WMAP5 ) and baryon acoustic oscillations .