How can I tell if a Mac App Store app is a universal purchase? Apple has announced that it is officially launching the ability for developers to sell Mac and iOS apps as a single purchase.
The macOS version of your app can now be included in a universal purchase, allowing customers to enjoy your app and in‑app purchases across iOS, iPadOS, macOS, watchOS, and tvOS by purchasing only once.
(source)

For example, GoodNotes 5 is a universal app, but I know it only through the linked support article. Its Mac App Store page only says the iPhone and iPad counterpart apps are available, but nothing about those being universal (or not).
The other app I'm looking at is iA Writer for Mac OS (but could be any other app, really), and unlike the Good Notes 5, I couldn't find any external mention of iA Writer being bundled with the iOS and ipadOS apps as a single purchase.
So can I somehow tell myself, without contacting the developers every time I need an answer, just by looking at the App Store pages, it's source code, or maybe making and API call of some sort?
I think I found the answer: you will have to find the app on both Mac and iOS App Stores, get the shareable link and compare the IDs. If those match, chances are it's a bundled app.