jQuery: check if a div contains a div with class on

if ($(".toDiv").find(".on").length > 0){ 
  ///do something
}

or

if ($(".toDiv .on").length > 0){ 
  ///do something
}

$('div.toDiv').each(function() {
    if($('div.on', this).length > 0) {
        //do something with this
    }
});