  Through studying the critical phenomena in continuum-percolation of discs, we
find a new approach to locate the critical point, i.e. using the inflection
point of $P_\infty$ as an evaluation of the percolation threshold. The
susceptibility, defined as the derivative of $P_\infty$, possess finite-size
scaling property, where the scaling exponent is the reciprocal of $\nu$ -- the
critical exponent of correlation length. The possible application of this
approach to the study of the critical phenomena in relativistic heavy ion
collisions is discussed. The critical point for deconfinement can be extracted
by the inflection point of $P_{\rm QGP}$ -- the probability for the event with
QGP formation. The finite-size scaling of its derivative can give the critical
exponent $\nu$, which is a rare case that can provide an experimental measure
of a critical exponent in heavy ion collisions.
