  We show how a test of macroscopic realism based on Leggett-Garg inequalities
(LGIs) can be performed in a macroscopic system. Using a continuous-variable
approach, we consider quantum non-demolition (QND) measurements applied to
atomic ensembles undergoing magnetically-driven coherent oscillation. We
identify measurement schemes requiring only Gaussian states as inputs and
giving a significant LGI violation with realistic experimental parameters and
imperfections. The predicted violation is shown to be due to true quantum
effects rather than to a classical invasivity of the measurement. Using QND
measurements to tighten the "clumsiness loophole" forces the stubborn
macrorealist to re-create quantum back action in his or her account of
measurement.
