Hi @stone1343. Here's my impression but I welcome any corrections. Things have been changing over the years.
The /etc/lsb-release file should contain the level the o/s is. Right now my old LTS says:
~$ cat /etc/lsb-release
DISTRIB_DESCRIPTION="Ubuntu 16.04.4 LTS"
This file comes in one of the core system packages. This install was done in May 2016 with the first 16.04 ISO and has only been updated via Synaptic.
But it gets a little more complex because the 16.04.4 ISO may differ for a planned reason - kernel level for "Hardware Enablement" (hwe). The 16.04.4 ISO installs a later kernel by using main kernel packages with the "hwe" designation. Normal updates will not switch kernels on you and this is how it goes:
1) A new install may benefit from a newer kernel because of newer hardware.
2) Current installations should simply be left alone in kernel version.
By "left alone" I don't mean security updates are ignored... quite the opposite. The older kernel is maintained with its own patches because it's an LTS. No issue there.
As for 18.04.1 I don't know for sure but it's likley too early to see a major kernel version update... perhaps that will start in 18.04.2 or 18.04.3.
Just my 2 cents.