client side checkbox validation in asp.net

.aspx

<asp:CheckBox ID="cbxTerms" runat="server" />
 <asp:CustomValidator ID="CustomValidator1" runat="server" ErrorMessage="error message" ClientValidationFunction="cbxvalidate">                                                 </asp:CustomValidator>

java script
 function cbxvalidate(sender,args)
{
            if (document.getElementById('chkbTerms').checked == false)
           {
                args.IsValid = false
            }
 }

Note
To call client side page validation manually from java script
Page_ClientValidate() //call this