How can I confirm if a message was unsent in iMessage?

This was using iMessage on my Mac. I sent a message to the wrong person (and it would be bad if they got it), but I was confused about how to unsend it. I just went to the message and deleted all the text. I quit and restarted iMessage and that message did not show. Also, in notifications, I had "unable to send".

However, when I looked in messages on my iPhone, that message still showed up. Now I have no idea if it was unsent or not.