32-bitCheck & ArchiChect

ArchiChect – check any app, bundle or file for 64-bit compatibility, signatures, notarization, and quarantine

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Updated! Drag and drop, or Open, almost any app, bundle, command tool, Installer package, even signed AppleScripts and regular documents, and ArchiChect will tell you whether its quarantine flag is set, if it contains executable code whether that is fully 64-bit, whether it has a valid signature, and whether it has been notarized, delivered by the App Store, or signed by Apple. The ultimate tool for checking Catalina compatibility, and much more. Version 2.4 requires 10.14 or later and provides full details of Universal Apps and binaries too.
ArchiChect 2.3 (Sierra, High Sierra, Mojave and Catalina)
ArchiChect 2.4 (Universal App for Mojave, Catalina and Big Sur)

32-bitCheck – checks installed apps, code bundles, command tools and more for those which are 32-bit

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32-bitCheck is a much better tool for checking which apps and other software are still 32-bit. Check the folders of your choosing, and can check just apps, or all bundles including plugins and other executable code. Generates text reports to help you prepare your Mac for macOS 10.14 later this year. Why waste time and effort using System Information? New version 1.8 can resize text and checks for updates automatically. See also ArchiChect below for simple drag-and-drop checking.
32-bitCheck 1.8 (El Capitan, Sierra, High Sierra, Mojave and Catalina)

Known issues:

ArchiChect 2.1 fixes a bug in which the busy spinner was hidden when busy, and improves reporting and text files.
ArchiChect 2.4 is recommended for Mojave and later, where it provides extensive information about Universal Apps and binaries. For previous versions of macOS, use version 2.3 instead.

Articles:

ArchiChect 2.4 goes Universal and improves detection of Universal code
Five Universal Apps: ArchiChect, DelightEd, LockRattler, SilentKnight and Ulbow
Taccy and ArchiChect now detect apps built for macOS 11.0 Big Sur
How to find all your 32-bit apps: a non-buyer’s guide
What’s in a macOS installer app? An exploration with ArchiChect
How to check quarantine, 64-bit, signature, and notarization for almost anything