Populating dropdown list in code behind

Question

I have this code in page.ascx-
<ItemTemplate>
            <asp:DropDownList id="dd" runat="server" >
            <asp:ListItem Value="Pending">Pending</asp:ListItem>
        <asp:ListItem Value="Yes">Yes</asp:ListItem>
        <asp:ListItem Value="No">No</asp:ListItem>
       </asp:DropDownList>
</ItemTemplate>
I want to check that if user has done some changes in the dropdown list say(changing from "pending" to "yes" or from "yes" to "no" anykind of change) then a dialog box should pop up to ask "do u want to notify the changes to the user"
How should i achieve this when populating in the code-behind ?