How to detect if any of my activity is front-most and visible to user?

I don't know that there's a method to get the currently displayed activity, but you could do something combining the Activity Lifecycle and a flag.

For the flag, if you've extended the Application class, that's probably a decent place to store it. For extending the application class, the top answer to this question has info. (d).

So probably keep track of the current active activity (or a flag that the activity is visible) in onResume/onPause or onStart/onStop depending on exactly what behavior you want.

Since you have multiple activities, you'll need a centroid place for storing the flag, which is why the Application makes sense. You can get the custom Application object by casting the application context (e.g. ((MyApplication)getApplicationContext()).isMyActivityActive).

You could extend Activity as well to help keep this code clean and contained.

If you're using a service you could bind to the service in every activity in the onStart/onStop (or onResume/onPause). If bound, you're visible.

You can ask for the running tasks from ActivityManager:

ActivityManager activityManager = (ActivityManager)getContext()
                .getSystemService(Context.ACTIVITY_SERVICE);
List<RunningTaskInfo> runningTasks = activityManager.getRunningTasks(Integer.MAX_VALUE));

From API docs:

Return a list of the tasks that are currently running, with the most recent being first and older ones after in order.

So the first item on the list is the frontmost activity.

onResume() called && onPause() not called = visible.
Have a public static Activity currentlyVisible; in your Application subclass that will be updated by your activities (set to the instance in onResume() and nulled in onPause()). Or invent a less ugly variant of a registry.

You could use onWindowFocusChanged(boolean hasFocus) place this in a superclass of your activities to launch the intent if it has focus.