how to check for a form submit validate result

how to check for a form submit validate result

Hello
I would like to add a jquery function to check the result of an existing forms submit validate result without modifying the exiting submit javascript function (customers function which i cannot change),

at the moment my function is called after the existing submit function, how can i check the result of the previous validation in this function?

$("form").submit(function() {
alert("changed in jquery ");

//NEED to check the result of customervalidationcheck() here
});


<form method="post" action="formcheck.html" onsubmit="customervalidationcheck()" name="testform" id="testform">

thanks
paul