  We introduce a novel approach to study the critical behavior of equilibrium
and non-equilibrium systems which is based on the concept of an instantaneous
correlation length. We analyze in detail two classical statistical mechanical
systems: the XY model and the Ising model, and one of the prototype models of
Self-Organized Criticality: the forest fire model (FFM). The proposed method
can both capture the critical behavior of the XY model and the Ising model and
discriminate between the nature of the phase transition in the two scenarios.
When applied to the FFM, it gives surprising results, suggesting that the model
could be critical despite displaying broken scaling in the distribution of
cluster sizes.
