I operate a small-medium fleet of macOS and OS X computers. For various legacy software and licensing reasons, the fleet operates on each release from 10.9 (Mavericks) to 10.15 (Catalina, current). All of these releases are free to download via the App Store and from Apple directly via their website.
I am not a member of any Apple program that provides access to early and/or special macOS / OS X builds. I am reliant on common sense and trial-and-error to maintain a set of OS installers from official sources. Sometimes the App Store installers are silently updated, especially when minor updates are applied outside of a combo updates release.
Right now, I check every couple of weeks and spend time downloading ~5-8GB, check the build number and invariably see it hasn't changed, then ditch the download. This seems wasteful.
I would like to be able to check a location online that can tell me the current build number of the active release, for example the one that's downloadable from the App Store.
Is there a location I can check that will tell me the build number of the freely-available macOS & OS X releases?
Edit #1: addressing @Jay-Way's post below with a screenshot. It looks like Mactracker doesn't have 10.15 (Catalina) details at all, so I'm guessing it's a manually-maintained list by the author or a third party.