Try this
HTML
<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
<title></title>
<style type="text/css">
body
{
font-family: Arial;
font-size: 10pt;
}
</style>
<script type="text/javascript">
function clientActiveTabChanged(sender, args) {
var selectedTab = sender.get_tabs()[sender.get_activeTabIndex()]._tab;
var tabText = selectedTab.childNodes[0].childNodes[0].childNodes[0].innerHTML;
alert(tabText)
}
</script>
</head>
<body>
<form id="form1" runat="server">
<div>
<cc1:ToolkitScriptManager runat="server">
</cc1:ToolkitScriptManager>
<cc1:TabContainer ID="TabContainer1" runat="server" OnClientActiveTabChanged="clientActiveTabChanged">
<cc1:TabPanel ID="TabPanel1" runat="server" HeaderText="Test1">
<ContentTemplate>
This is Tab1</ContentTemplate>
</cc1:TabPanel>
<cc1:TabPanel ID="TabPanel2" runat="server" HeaderText="Test2">
<ContentTemplate>
This is Tab2</ContentTemplate>
</cc1:TabPanel>
<cc1:TabPanel ID="TabPanel3" runat="server" HeaderText="Test3">
<ContentTemplate>
This is Tab3</ContentTemplate>
</cc1:TabPanel>
</cc1:TabContainer>
</div>
</form>
</body>
</html>