How do you see any messages that are being backed up to the cloud?

If you are using messages in iCloud, your messages are not 'backed up' to the cloud.. they reside in the cloud for syncing to other devices. If you delete a message from any device that's synced with iCloud, you are also deleting that message from iCloud.

You'd see whatever messages you have by looking on your phone or whatever devices are using messages in iCloud on the same Apple ID. You cannot go to iCloud.com and view them or anything like that.

If you are not using messages in iCloud, and you're referring to an iPhone backup, you cannot dig thru the contents of the backup. In order to see exactly what's in that backup, you'd have to erase and restore your iPhone with that backup.

Use Messages in iCloud - Apple Support

Backup methods for iPhone, iPad, and iPod touch - Apple Support

Restore your iPhone, iPad, or iPod touch from a backup - Apple Support

You can't. Assuming you are backing up to iCloud, the messages that you see on your phone are the messages that are being backed up to iCloud.

In other words, your backup is simply a copy of your phone. If you delete a message from your phone, the same message will be deleted on the next backup that runs.