On most of their plugins (if you look hard enough) you can usually find in tiny print somewhere in the Inspector or on viewer pane some ambiguous numbers that indicate the version. E.g, for Core Melt's Lock-n-Load it is in the Inspector in parenthesis right under the name. For DriveX it is at the bottom of the FCPX viewer in tiny print.
Unlike macOS apps which mandate the version number in the "About" menu, there is no equivalent standardization for FCPX plugins. The plugins exist in a more fluid, less defined UI environment. There are no fixed menus.
However an old or malfunctioning plugin can crash FCPX and stop your work, so it's just as important you know the version as for macOS apps.
As FCPX.guru said, it's sometimes just easier to update the plugins. FxFactory is like an app store for FCPX plugins and they have an auto update procedure. fxfactory.com
But many important FCPX plugins aren't available in FxFactory and they all handle versioning and updates differently. It is like the "wild west". It is roughly at the point Windows apps were in 1985. This is a fundamental shortcoming of the FCPX plugin system. You won't see this listed on most "top 50 requested FCPX enhancements" because most people don't think in those terms. However improvement in this area is badly needed.
Hi, the build number is displayed either in the inspector or the on screen controls. As we do continuous updates we switched to a build number system rather than version numbers. (507) is the latest for all plugins except Lock and Load which is version (160). We do notify customer of updates on our email list and our facebook page and elsewhere. We will be improving our update system shortly.
You are correct Apple doesn't really give a good way for us to manage versioning of plugins, however we can still improve on what we have now and are working on that.
PS Latest versions do work on Catalina, we will have notarized installers very soon