Stop User From Leaving Form Before Complete?

I am having trouble with a page that has a form on it. What I need to be able to do is to pop up a message on the page (alert or confirm function) that will tell the user that leaving this page is unwise as their information will not be sent. Here is the tricky part:

1. This page has a lot of links on it that can take the user to any number of different pages. I am not able to put any javascript additions to the html of the links.

2. I don't want the message to pop up when the user clicks the Finish button ( a submit or button form input, whichever works best ).

I have added and onunload() javascript to the body of my page so that when any link is pressed (including the finish button) it runs a script to verify the pressing of the finish button. It SHOULD be checking to see if Finish was pressed and if it was continue, if not issue the confirm or alert to warn the user that they should not leave before finishing.

I have already been through the javascript O'reilly guide and can't figure it out. In PHP (which is my primary language) I could say something like:

if (!isset($finishbutton)) { do what I want };

However, PHP won't work for this. I need to be able to have JS do something like this:

if ({formelement finishbutton} has NOT been clicked) { do what I want }