  We present a novel spectroscopic method for probing the \insitu~density of
quantum gases. We exploit the density-dependent energy shift of highly excited
{Rydberg} states, which is of the order $10$\MHz\,/\,1E14\,cm$^{\text{-3}}$ for
\rubidium~for triplet s-wave scattering. The energy shift combined with a
density gradient can be used to localize Rydberg atoms in density shells with a
spatial resolution less than optical wavelengths, as demonstrated by scanning
the excitation laser spatially across the density distribution. We use this
Rydberg spectroscopy to measure the mean density addressed by the Rydberg
excitation lasers, and to monitor the phase transition from a thermal gas to a
Bose-Einstein condensate (BEC).
