  We introduce a new method for determining the critical indices of the
deconfinement transition in gauge theories. The method is based on the finite
size scaling behavior of the expectation value of simple lattice operators,
such as the plaquette. We test the method for the case of SU(3) pure gauge
theory in (2+1) dimensions and obtain a precise determination of the critical
index $\nu$, in agreement with the prediction of the Svetitsky-Yaffe
conjecture.
