You can open the window using such code:
<script type="text/javascript"> var oWindow = 0; function OpenWin(sURL) { oWindow = window.open(sURL, "_blank"); } </script>
This will open the window and save reference in global variable.
If the new page is in your own domain and the form submits to different page, you can check the oWindow.location.href in timer e.g. every half a second, and if the location match the form action you can assume it was submitted.
If the new page is in your domain you can also hook into the form onsubmit, using jQuery is the most simple way - can you use it?