Posting to Slack failed; how can I check?

I posted a new Topic; I expected a Slack message; I waited 20 seconds; I saw nothing.

Where can I look to differentiate “posting to Slack failed” from “Discourse failed to try to post to Slack”? I found an old thread about error logs, but it seems out of date; I found the error logs and there is no entry there. This leads me to believe that Discourse failed to try to post to Slack.

This has worked in the past. The Slack app is in the right channel. The Slack permissions are correct, as far as I can tell.

All suggestions are welcome.