How does iphone6 know if it has sent an imessage?

If you are sending an iMessage message, the message you send will have a gray mark in the lower right corner. If it is an ordinary short message, there is no reminder, only whether the mobile has sent a successful reminder.

Extended answer:

If the message has been sent: imassage will be displayed before the first imessage, and the dialog box replied by the other party will be displayed in blue (green for ordinary SMS).

If you are editing a short message: If you can send an imessage when there is no text in the input box, the words imessage in gray will be displayed in the input box.

The blue dialog box is imessage, and the green one is ordinary SMS. If you want to cancel, just cancel in the SMS settings.