A few things happened in 2018. First, MCU was upgraded from MCU1 to MCU2 starting mid-March, give or take. Then approximately starting mid summer (possibly late Aug), the Premium Upgrade Package & Upgraded Stereo became standard. Prior to this point, those two were options, and were not on all X's.
When you get into the car, touch the Car icon in the lower left. Go into Software, and click Additional Vehicle Options. That will tell you:
Base Audio or Premium Audio
Tow Package?
Cold Weather Package?
MCU Processor (Nvidia Tegra = MCU1, Intel Atom = MCU2
To find out if it has the Premium Upgrade Package, go into HVAC controls. If it has Bioweapons Defence Mode, then it has it.
This page may help, although it doesn't have specific dates, just years.
Tesla history of model changes by year, we list all the hardware changes and updates to each model
tesla-info.com