Bare bones, but confirmed working, example:
<script type="text/javascript">
var clicked;
function mysubmit() {
alert(clicked);
}
</script>
<form action="" onsubmit="mysubmit();return false">
<input type="submit" onclick="clicked='Save'" value="Save" />
<input type="submit" onclick="clicked='Add'" value="Add" />
</form>