How do I tell if my current thread is the UI thread?

My son just encountered this as an Issue so I thought I would add an updated answer.

The Main UI thread can be accessed using the Core Dispatcher CoreWindow.GetForCurrentThread().Dispatcher or Windows.ApplicationModel.Core.CoreApplication.MainView.CoreWindow.Dispatcher

Below is a class that implements the Core Dispatcher and tests whether the calling thread is the Main UI Thread. If so it invokes the action, otherwise calls Dispatcher.RunAsync to execute it on the Main UI thread.

class Threading {

     private static CoreDispatcher Dispatcher => 
     Windows.ApplicationModel.Core.CoreApplication.MainView.CoreWindow.Dispatcher;

     public static async void ThreadSafe(DispatchedHandler action)
        {
            // Calls Dispatcher.RunAsync to run a method on the Main UI Thread
            IAsyncAction UiThread(DispatchedHandler proc) => Dispatcher.RunAsync(CoreDispatcherPriority.Normal, proc);

            // Checks to see if this was called from the Main UI thread 
            // If we are in the Main UI thread then Invoke the action 
            // Otherwise: Send it to run in the Main Ui Thread.

            if (Dispatcher.HasThreadAccess) { action.Invoke(); } else { await UiThread(action); };
        }
    }

The above class could be used like so:

// Some event handler callback
private void SomeCallback(object sender)(){
    void line() => Frame.Navigate(typeof(PageName));
    Threading.ThreadSafe(line);
}

// Pass a handle to the control and a string to update it's text property
internal static void ChangeControlText(dynamic ctrl , string v)
    {
                void line() {ctrl.Text= v;}
                ThreadSafe(line);
    }

I have found the solution...

CoreDispatcher.HasThreadAccess returns a bool indicating if you are on the UI thread or not.