No easy way that I know of. Yeah, I guess AppOfflineError generally won’t be recorded in the app logs, now that I think of it, because that’s an error that occurs on the client-side and means that it can’t contact the Anvil server (including to register to event in the logs). It is the error that is triggered when the client device loses its connection to the internet, but I do think it can also be triggered if the internet connection is fine in general but the Anvil servers are unavailable in particular.
If you’re troubleshooting it live with someone, you can ask whether the internet is working for other websites, and you can check whether the Anvil servers are working otherwise. But I’m not sure if there’s a way to sort out a past event in retrospect, other than checking the forum to see if others reported an Anvil outage.
update: This clever new feature request might make it possible to see some such errors in app logs: