Critical current is one of the important properties of superconductors. So, how to calculate or estimate the critical current density of superconductor room AC susceptibility measurement.
The most simple formula for an infinite slab in a parallel magnetic Hac field is: Jc=2Hac/d where d is the thickness of the slab and Hac is the amplitude of alternating magnetic field. Here Hac field corresponds to full penetratation field (when the magnetic flux enters the centre of the sample) and consequently to maximum of looses chi".
If you measure chi"(T) at constant Hac at some temperature chi" attains maximum and Jc=2Hac/d obeys (for given temperatrure and Hac).