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.