  Recent developments of the Josephson plasma resonance and transport c-axis
measurements in layered high T$_{c}$ superconductors allow to probe Josephson
coupling in a wide range of the vortex phase diagram. We derive a relation
between the field dependent Josephson coupling energy and the density
correlation function of the vortex liquid. This relation provides a unique
opportunity to extract the density correlation function of pancake vortices
from the dependence of the plasma resonance on the $ab$-component of the
magnetic field at a fixed $c$-axis component.
