  We derive an effective coupling parameter for two-dimensional Yukawa systems
based on the height of the first maximum of the pair distribution function. Two
variants -- one valid in the high-coupling range, the other for arbitrary
couplings of the liquid -- are derived. Comparison to previous approaches to
Yukawa coupling parameters shows that the present concept is more general and
more accurate.
  Using, in addition, dynamical information contained in the velocity
autocorrelation function, we outline a reference data method that can be
employed as a non-invasive measurement scheme of the plasma parameters -- the
coupling strength and the screening length. This approach requires only input
from a time-series of configuration snapshots and particle velocities with no
recourse to additional information about the system. Our results should be
directly applicable as a simple, yet reliable diagnostic method for a variety
of experiments, including dusty plasmas, colloidal suspensions and ions in
traps, and can be employed to facilitate comparisons between experiments,
theory and simulations.
