Hi,
I was also quite ignorant regarding ATFs.

Mine is a 722.4 with a dipstick. The mechanic checks the fluid with the engine running, In Park and the engine quite warm.

I thought it was like checking engine oil wherein it can be done when the engine is shut down.

My car's mileage is in the 115,000 kms plus. The MB dealer says I should drain and change the fluid every 100,000 kms. I checked the fluid condition and it is still very clear. It says nothing about trans. fluid change or any service in the owners manual.

BTW. What ATF are you using for the car. I have a1996 E-230.
Some say using a wrong type of atf is bad for the gears.

Thanks.