C# Attach event to HTMLElement in webBrowser

gistfile1.cs

        /// <summary>
        /// Inject onclick handler to close button
        /// </summary>
        private void InjectJS()
        {
            HtmlDocument doc = webContent.Document;
            HtmlElement closeBtnElement = doc.GetElementById(CloseButtonId);

            if (closeBtnElement == null)
                return;

            closeBtnElement.AttachEventHandler("onclick", new EventHandler(WebCloseButtonCallBack));
        }

        /// <summary>
        /// Called when close button is clicked
        /// </summary>
        /// <param name="sender"></param>
        /// <param name="e"></param>
        private void WebCloseButtonCallBack(Object sender, EventArgs e)
        {
            //Do something here
        }

This is nice. But how to detect if an event listener is already attached.