I didnt try but first you can check if notifications are disabled by using NotificationManagerCompat.areNotificationsEnabled(), from support library. The versions below API 19 will return true (notifications are enabled).
Then if notifications disabled, you can warn your user and start notification settings like this answer:
Intent intent = new Intent(); intent.setAction("android.settings.APP_NOTIFICATION_SETTINGS"); //for Android 5-7 intent.putExtra("app_package", getPackageName()); intent.putExtra("app_uid", getApplicationInfo().uid); // for Android 8 and above intent.putExtra("android.provider.extra.APP_PACKAGE", getPackageName()); startActivity(intent);