My front drivers side window goes down quietly, but will have some creaking/popping sounds when rolling up. How would I know if it’s a bad regulator or window motor without taking apart the door panel? Are window regulators a common failure item on w212’s?

 

you would need to take off door panel and inspect.
Heck it could be just mounting hardware is lloose or regulator needs a bit of lubricant.

See going down is easier but when going up bit more forces in all those regulator pin joints and slides and other moving parts

Maybe first is roll window down all the way and clean the rubber channels where window rides very good with some good rubber cleaner and then when dry apply some door seal protectant in groove ...Gummi Phledge could be one to use.
OR check out Griots I think they have one too... and probably every other detailing supply company.

Thing is do not get one that is Oily but more water like and you apply and let sit for few mins then buff with clean old towel be it terry cloth or Micro fiber.

Then try window up and down again... did it get better?
If yes but not perfect then remove door panel and check mounting hardware is tight and maybe clean off old nasty lube you find and re-lube what you can.