출처

(function($){
     $.extend({
       isRuleExist:function(className){
              var classes = [];
              var sheets = document.styleSheets;
              var push = Array.prototype.push;
              for(var i = 0,n = sheets.length; i < n; i++){
                push.apply(classes, sheets[i].rules || sheets[i].cssRules);
              }
              for(var i = 0,n = classes.length; i < n; i++){
                 if(classes[i].selectorText == className){
                         return true;
                 }
             }
             return false;
          }
     });
})(jQuery);

사용법