  We consider the problem of performing the preliminary "symmetry
classification'' of a class of quasi-linear PDE's containing one or more
arbitrary functions: we provide an easy condition involving these functions in
order that nontrivial Lie point symmetries be admitted, and a "geometrical''
characterization of the relevant system of equations determining these
symmetries. Two detailed examples will elucidate the idea and the procedure:
the first one concerns a nonlinear Laplace-type equation, the second a
generalization of an equation (the Grad-Schl\"uter-Shafranov equation) which is
used in magnetohydrodynamics.
