How do I know if an iMessage was delivered?

If this is an iMessage (blue) which means it has been sent to another Apple device that's logged into iMessage, you'll see "Delivered" under the most recent message in the thread. You will not see that under earlier messages.

If this is an SMS (green) message, you will only see a status message if it was not delivered. As SMS messages will go through when almost nothing else will, it is generally safe to assume that it was delivered.

If you're asking about FaceBook Messenger, you'll need to ask Facebook. Their software for iOS and iPadOS particularly, has been awful of late.

If That’s Facebook messenger and isn’t a Apple product or service and if you send from the Apple en message app to a iPhone iPad of Mac you can see delivered under the message but that doesn’t work with androids phones