I’m no expert on these things but I’d think for the vast majority of cases, all you’d be interested in is whether a minimum version is installed, so wouldn’t you want to check that rather than if it was the system Ruby?
Posting here to lobste.rs for a check of accuracy. Surprisingly (or not), beginners often ask, “Is Ruby already installed on my Mac?” So trying to answer that and provide a link to an installation guide I’ve written.
Ruby has been installed on macs since at least OSX 10.4
It’s probably out of date, and most guides want you to install a newer copy, but it’s always there, and is a good starter ruby, if you just want to play around with the language. If you want a newer copy you can always use rvm or rbenv to install and use a newer version, and not mess with the system installed ruby.