How do I identify the apps and programs on my Mac that are using Legacy Extensions?

That is a very good question. At this time, so far as I know, the best way would be to create a list of apps which are identified from the periodic Apple notifications. That said, however, by the time that the new OS is released, many of those on the list may have been updated and become compatible. In the alternative, when we get update notifications from those vendors, after installing them we could visit the vendor's web site and ask them if they are compliant. Most vendors maintain lists of which OS's they are compliant with. In the end I suspect that there will likely be very few that are no longer compliant.