You would use JavaScript code similar to the following (assuming your WebTab instance has a server-side ID of "UltraWebTab1"):

var tab = igtab_getTabById("<%= UltraWebTab1.ClientID %>");
var idx = tab.getSelectedIndex();

You can find more information in the WebTab CSOM Overview (link leads to our online help documentation of NetAdvantage for .NET 2008 Volume 3).