using jquery you can do it in this way.
<script type="text/javascript">
$(document).ready(function () {
$("#Button2").click(function () {
alert(checkList());
});
});
function checkList() {
var flag = true;
$("input[id*='CheckBoxList1']").each(function () {
if (!$(this).is(":checked"))
flag = false;
});
return flag;
}
</script>
<div>
<asp:checkboxlist id="CheckBoxList1" runat="server" xmlns:asp="#unknown">
<asp:listitem>select all</asp:listitem>
<asp:listitem>a</asp:listitem>
<asp:listitem>b</asp:listitem>
<asp:listitem>c</asp:listitem>
<asp:listitem>d</asp:listitem>
</asp:checkboxlist>
<input id="Button2" type="button" value="button" />
</div>