How to fire jQuery function only if form is valid

If you are using jquery validate unobtrusive validation you could:

$(function () {
    $('#signupform').submit(function () {
        if($(this).valid()) {
            alert('the form is valid');
        }
    });
});

$(function () {
    $('#signupform').submit(function (e) {
        if (validateForm() === true) {
            alert('Form is valid.');
        }
    });
});

Note, the validateForm() function would be the one you use within the browser to test validation. EDIT: This is indeed jQuery's $(this).valid().

NOTE: On clarification of the question, the answer above is one way to stop form submitting on validation, but not the technique that was desired (jQuery valid() validation). See Darin Dimitrov's answer for details.

Also, on rereading the original question, the intent was the exact opposite of what I originally described. So I edited to demonstrate a functional response on validation, not how to prevent submitting a form.