How can I tell if a Mac App Store app is a universal purchase?

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.

UPD: The next day after posting this, iA Writer Support sent me a reply which doesn't conflict with my above theory:

If I buy the app on the Mac App Store, is it a universal purchase and are the iOS counterparts for iPhone and iPad bundled with it? Or will I have to purchase them separately?

The software licenses for iA Writer are platform specific requiring a separate purchase for each. All of our apps are developed independently and unlike subscription services or login based apps, you own iA Writer on whichever platform you've purchased it for. 🙂

Use of iA Writer on iOS would require purchasing iA Writer for iOS available here: iA Writer for iOS