  Although nonclassical quantum states are important both conceptually and as a
resource for quantum technology, it is often difficult to test whether a given
quantum system displays nonclassicality. A simple method to certify
nonclassicality is introduced, based on easily accessible collective atomic
quadrature measurements, without the need of full state tomography. The
statistics is analyzed beyond the ground-state noise level, by direct sampling
of a regularized atomic quadrature quasiprobability. Nonclassicality of a
squeezed ensemble of 2x10^5 Cesium atoms is demonstrated, with a significance
of up to 23 standard deviations.
