Honestly- subjectively. Does the software crash often, do I see weird behaviour as a user, what's the number of users, bug report rates. I think those systems

Honestly- subjectively. Does the software crash often, do I see weird behaviour as a user, what's the number of users, bug report rates. I think those systems do compare in complexity to at least pieces of todays software. A lot of this older code is still powering things today/morphed into newer/larger systems. I've worked in more recent times on software that was used by millions of people, was extremely reliable, and didn't have a lot of unit tests. We did have other forms of automated testing though.