Find the app online (Google for the iTunes link). For this example, we use Apple Pages.
Copy the number after the ID in the URL (here it’s 361309726).
Open https://itunes.apple.com/lookup?id=361309726, replacing the ID with the one you looked up.
Search the output for bundleID. In this example, it looks like this (next to a bunch of other data): "bundleId":"com.apple.Pages" So for Apple, the bundle ID is com.apple.Pages.