Validate the "-Select Item-" in a bound DropDownList

Hi Friends,
I have a data bound dropdl and I already added a new item in the following way:
            ListItem li = new ListItem("--Select a Counselor--", "0");
            cbCounselor.Items.Add(li);
            li.Selected = true;

 What I want to do now is validate that the user selected a valid item in the DDL. How can I do that? What is the validator I need to use here...and how?
 
Thanks a lot 
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