How can I tell if a MacOS App is notarized or sandboxed?

I'm not sure if this helps but if you open System Information and in the side bar, go down to Software > Applications, you should see all the applications installed on your Mac. Clicking on any app will show you their certificates, version, and where you obtained these applications (i.e. from Mac App Store, Apple, identified developer or unknown).

To access System Information you can open it from a Spotlight Search, or click on the About this Mac in the Apple Menubar, and click on System Report.

Hope this helps.

You can tell this with taccy from the Eclectic Light Company. It reports directly on whether an app is notarized (although I find its "Developer ID" checkbox behavior confusing -- it doesn't check it if the app is notarized, even though notarized apps are developer-ID signed).

taccy doesn't explicitly report whether the app is sandboxed, but you can tell from the detailed display. If the "Entitlements" section says "Property list incorrect format", that seems to indicate it's not sandboxed. Here's an example of its report on an old (pre-sandbox and notarization) version of Firefox: