You cannot reliably see an unsent iPhone message after it has been unsent in Messages
Check the notification preview on your lock screen or notification center if it appeared before being unsent
Check your Apple Watch if message notifications were mirrored there
Check any other Apple devices signed into the same Apple ID for a notification preview
Ask the sender to resend the message
If you use a third-party messaging app, check that app’s chat history or notification history if available
Check whether your iPhone has a notification log feature through a third-party app or device management tool
Restore from an iCloud or computer backup made before the message was unsent if you need the original content
Contact the sender directly if the message is important