How to validate an Ajax control combo box has item selected or not?

First, you need to insert beside the combo box a custom validator like this,

<asp:CustomValidator ID="cv" runat="server" ControlToValidate="cmb" Text="*" ForeColor="Red" ValidateEmptyText="true" ClientValidationFunction="CustomValidator" Display="Dynamic" ErrorMessage="combo box is required!" CssClass="validator" ValidationGroup="valGrp"></asp:CustomValidator>

If your combo box default selected value is an empty string then you need to add a function inside a <script type="text/javascript"> like this,

    function CustomValidator(s, e) {

        if ($find(s.controltovalidate)._textBoxControl.value == "") {

            e.IsValid = false;

        }

    }

If your combo box default value of “Please select” then you need to add a function inside a <script type="text/javascript”> like this,

    function CustomValidator(s, e) {

        if ($find(s.controltovalidate)._textBoxControl.value == "Please select") {

            e.IsValid = false;

        }

    }

References used:

http://blog.csdn.net/ywuwgij/article/details/4617140