How do you make this field required if the above checkbox is checked?

How can I make lblLineoutReason field required ONLY if the chkLineOut checkbox is checked??

 <asp:HyperLink ID="lblLineOut" runat="server" Text="Line Out Record:" CssClass="labelText" Width="140px"></asp:HyperLink>&nbsp;
                            <asp:CheckBox ID="chkLineOut" runat="server" CssClass="chkBox" Enabled="false" />&nbsp;

                            <asp:HyperLink ID="lblLineOutReason" runat="server" Text="Reason:" CssClass="labelText" Width="100px"></asp:HyperLink>&nbsp;
                                <asp:TextBox ID="txtLineOutReason" runat="server" CssClass="textboxDisabled" ReadOnly="true" Width="490px" /><br />