Check if user has granted NotificationListener access to my app

The only correct way to do it is to check the secure settings:

ComponentName cn = new ComponentName(context, YourNotificationListenerService.class);
String flat = Settings.Secure.getString(context.getContentResolver(), "enabled_notification_listeners");
final boolean enabled = flat != null && flat.contains(cn.flattenToString());

I was wrong, checking if service is running works:

    private boolean isNLServiceRunning() {
        ActivityManager manager = (ActivityManager) getSystemService(Context.ACTIVITY_SERVICE);

        for (RunningServiceInfo service : manager.getRunningServices(Integer.MAX_VALUE)) {
             if (NLService.class.getName().equals(service.service.getClassName())) {
                  return true;
             }
        }

        return false;
    }

If you're using androidx you can use

NotificationManagerCompat.getEnabledListenerPackages(Context context)

to check if your listener is enabled. Source