Populating dropdown list in code behind

[@kriti]

My code list.ascx has-

<ItemTemplate>
<aspropDownList id="dropdown" runat="server" Width="87px" FontSize="XX-Small" Font-Names="Verdana">
<asp:ListItem Value="Pending">Pending</asp:ListItem>
<asp:ListItem Value="Yes">Yes</asp:ListItem>
<asp:ListItem Value="No">No</asp:ListItem>
</aspropDownList>
</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 ?