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);