  The challenge of demonstrating that the matter produced in heavy ion
collisions is a deconfined quark-gluon plasma, as predicted by lattice QCD
calculations, is the challenge of measuring the number of thermodynamic degrees
of freedom \nu ~ \epsilon/T^4 at the time t0 at which the matter comes into
approximate local thermal equilibrium and begins to behave like a hydrodynamic
fluid. Data from experiments done at the Relativistic Heavy Ion Collider have
been used to estimate t0 and to put a lower bound on the energy density
\epsilon(t0). However, measuring \nu has seemed out of reach, because no
current data serve even as qualitative proxies for the temperature T(t0). We
point out that \nu may equally appropriately be defined via \nu ~s^4/epsilon^3,
where s is the entropy density, which can be estimated from the measured final
state entropy. This estimate is based on the testable assumption of an
isentropic expansion. The observation of jet quenching has the potential to
provide an upper bound on the energy density at early times. Our goal is to
motivate such an analysis by pointing out that it would set a lower bound on
\nu.
