In general, how to prove a variety has rational singularities intrinsically? i.e., don't use the Artin's criterion concerning the exceptional locus. And what kinds of varieties have only rational singularities? I've read a proof of classification of normal cubic surfaces (paper of Bruce and Wall), but their method is quite concrete, is there any way to prove the fact that a normal cubic surface which is not a cone can only contain rational singularities without using the classification of du Val singularities?