How to do something BEFORE a view is updated via ajax submit button?
You have to capture the mousedown event on the submit / apply button, prevent submission using e.preventDefault(), validate and if everything is fine submit the form using $('#form-id').submit()

Update:

Here is a fully tested and working code:

(function ($) {
    Drupal.behaviors.ValidateAjaxForm = {
        attach : function (context, settings) {

            var $form = $("form#views-exposed-form-list", context); // ajax form selector, or better specify by class name
            var $submitButton = $('#edit-submit-list', $form); // ajax submit button selector, or better specify by class name

            // Use off to remove any existing submit handler
            $form
            .off('submit')
            .on('submit', function (e) {
                e.preventDefault();
                var $form = $(this);

                var valid = false;

                // Run some validation on $form object.
                // check conditions here and if all is fine declare valid = true

                // If vaild, trigger event on button
                if (valid) {
                    $form[0].submit();
                }

            });
        }
    };
}
    (jQuery));