How to discover how much space a text (Label) will need

I have a fixed space on my page to print a text that can be just few words or a very long one. So I need to display part of it and add a "Show more" in case there's some text left. This is the HTML part (where Lbl_Desc and Lbl_FullDesc are populated in code behind):

                    <asp:Panel ID="Panel_HalfDesc" runat="server">
                        <div style="overflow-x: auto; overflow-y: hidden; height: 200px;" id ...