TOPIC: Alert for missing required fields?

Anyone know of an easy way to do a pop up alert when someone tries to submit a form with missing fields? As it is now, when someone is missing a required field and they hit submit, they do not always realize the problem is with a missing required field and then get frustrated the form is not working. This is especially true with long forms where the missing field may not even still be visible on the page without scrolling. What I would like to try to do is some type of more visual cue around the Submit button that the form is not complete... either a simple javascript popup alert or a notice right by the submit button that indicates as such.