Loop through the form fields and check if there is any field/s still left out empty, if yes keep disabled submit button, else enable submit button.
var flag = true;
$('#yourFormID').each(function()
{
if($(this).val() == "")
{
/* $('input[type="submit"]').attr('disabled'); may be you will need this if you are checking form on every form element*/
flag = false;
}
}
if(flag) {
$('input[type="submit"]').removeAttr('disabled');
}
I will suggest to call this code on on last element of your form