In the LINE app chats, I was offline when sending a group of photos. Now I am online and press the swirl and choose "resend".
The time appears, meaning the resend worked, but my friend says he only got one photo! I am at my wit's end.
P.S., you also asked "How to detect." Well, there is no way to detect it. Just keep a healthy suspicion and double-check all pictures were sent each time as per the steps above.