Using a background thread and Windows API's in Outlook, we can detect when the user has opened the Print tab in the backstage

DetectUserPrintInOutlook.cs

        [DllImport("user32.dll")]
        public static extern IntPtr FindWindow(string lpClassName, string lpWindowName);
        [DllImport("user32.dll", SetLastError = true)]
        public static extern IntPtr FindWindowEx(IntPtr parentHandle, IntPtr childAfter, string className, string windowTitle);
        /// <summary>
        /// Startup for Outlook Add-in
        /// </summary>
        /// <param name="sender"></param>
        /// <param name="e"></param>
        private void ThisAddIn_Startup(object sender, System.EventArgs e)
        {
            // we start by creating a background thread and look for a specific
            // set of windows to appear, then we know the user clicked print
            new Thread(() =>
            {
                while (true)
                {
                    Thread.Sleep(1000);
                    CheckForPrint();
                }
            }).Start();
        }

        /// <summary>
        /// Checks to see if the user has opened backstage and 
        /// selected the Print tab
        /// </summary>
        private void CheckForPrint()
        {
            try
            {
                // depending on whether we have an inspector active, or the explorer
                // active we will need to get the caption to FindWindow
                string LstrCaption = "";
                if(Application.ActiveWindow() is Outlook.Inspector)
                {
                    // Active inspector caption
                    LstrCaption = ((Outlook.Inspector)Application.ActiveWindow()).Caption;
                } 
                else if(Application.ActiveWindow() is Outlook.Explorer)
                {
                    // Active explorer caption
                    LstrCaption = ((Outlook.Explorer)Application.ActiveWindow()).Caption;
                }
                // get the window handle
                IntPtr LintHostHandle = FindWindow(null, LstrCaption);
                if (LintHostHandle == IntPtr.Zero) return; // if we cannot find it - nevermind
                // create a list of windows to find (in reverse order)
                // 4) rctrl_renwnd32 - is the print preview window
                // 3) NetUICtrlNotifySink - is whole Print options and preview
                // 2) NetUIHWND - is the the entire print tab
                // 1) FullpageUIHost - is the backstage page
                Stack<string> LobjWindowClasses = new Stack<string> (
                    new string[] { "rctrl_renwnd32", "NetUICtrlNotifySink", "NetUIHWND", "FullpageUIHost" });
                // recursive call back to find each window in the stack.
                // if all of them are found, then present a message to the user
                if(FindWindowStack(LintHostHandle, LobjWindowClasses))
                {
                    MessageBox.Show("You have clicked on the Print Tab in Outlook.");
                }
            }
            catch { }
        }

        /// <summary>
        /// RECURSIVE
        /// This function will take the window classnames in the provided stack
        /// and then find each one in order via recursive calls. If all of them
        /// are found - we return true = found
        /// </summary>
        /// <param name="PintHandle"></param>
        /// <param name="PobjStack"></param>
        /// <returns></returns>
        private bool FindWindowStack(IntPtr PintHandle, Stack<string> PobjStack)
        {
            try
            {
                // get the window with the classname being popped off the stack
                IntPtr LintNewHandle = FindWindowEx(PintHandle, IntPtr.Zero, PobjStack.Pop(), "");
                if(LintNewHandle != IntPtr.Zero && PobjStack.Count == 0)
                {
                    return true; // found it
                }
                else if(LintNewHandle!= IntPtr.Zero)
                {
                    // found a window, but the stack still has items, call next one
                    return FindWindowStack(LintNewHandle, PobjStack);
                }
                else
                {
                    // did not find it
                    return false;
                }
            }
            catch
            {
                // oops
                return false;
            }
        }