For any given version or installation of Linux Mint, how would I find out which version of Ubuntu it is based on? I'm sure it must be in documentation somewhere right?
You'll find Ubuntu version in the
$ cat /etc/upstream-release/lsb-release DISTRIB_ID=Ubuntu DISTRIB_RELEASE=14.04 DISTRIB_CODENAME=trusty
To figure out which subrelease you are using, you need to know what kernel you are running, e.g. here kernel 3.19:
$ uname -r 3.19.0-32-generic
Then you compare it with the 14.04.x Ubuntu Kernel Support schedule which says that in my case, the 3.19 kernel matches 14.04.3.