If you run a Mastodon server, you can easily add a piece of custom CSS which will automatically put a red border around image, video or audio posts that are missing a text description.
The custom CSS code is here:
➡️ https://gist.github.com/FiXato/3de505b04efefd49a1e3568b70545234
You can add it by logging into your server website and then going to Preferences > Administration > Site Settings > Appearance, paste the code into the Custom CSS box and click "Save Changes".