How can I verify that an iOS app has or has not been updated?
You can check the version of Messenger installed by going here:

Settings > General > Storage (&amp; iCloud Usage) > Manage Storage > Messenger
Version will be displayed next to app icon.

And you can check the latest version of Messenger in the App Store by going to its page and check the version.

If they are the same, you are good to go. If not, I suggest closing App Store, restarting your iPhone and then checking App Store. It will probably says Update and you can do so.