  The discovery of van der Waals superconductors in recent years has generated
a lot of excitement for their potentially novel pairing mechanisms. However,
their typical atomic-scale thickness and micrometer-scale lateral dimensions
impose severe challenges to investigations of pairing symmetry by conventional
methods. In this report we demonstrate a new technique that employs
high-quality-factor superconducting resonators to measure the kinetic
inductance -- up to a part per million -- and loss of a van der Waals
superconductor. We analyze the equivalent circuit model to extract the kinetic
inductance, superfluid stiffness, penetration depth, and ratio of imaginary and
real parts of the complex conductivity. We validate the technique by measuring
aluminum and finding excellent agreement in both the zero-temperature
superconducting gap as well as the complex conductivity data when compared with
BCS theory. We then demonstrate the utility of the technique by measuring the
kinetic inductance of multi-layered niobium diselenide and discuss the limits
to the accuracy of our technique when the transition temperature of the sample,
NbSe$_2$ at 7.06 K, approaches our Nb probe resonator at 8.59 K. Our method
will be useful for practitioners in the growing fields of superconducting
physics, materials science, and quantum sensing, as a means of characterizing
superconducting circuit components and studying pairing mechanisms of the novel
superconducting states which arise in layered 2D materials and
heterostructures.
