SpiritAbsolute,
<!DOCTYPE HTML>
<html>
<head>
<title>Untitled</title>
<script src="http://ajax.googleapis.com/ajax/libs/jquery/1.11.0/jquery.min.js"></script>
</head>
<body>
<ul id="tg-menu">
<li></li>
<li></li>
<li class="ui-menu-item ui-state-focus"></li>
</ul>
<script>
var i = $('#tg-menu > li').index($('li.ui-menu-item.ui-state-focus')) ;
alert(i)
</script>
</body>
</html>