as far as checking the level there is no easy way to do it. BMw doesnt makea dipstick for transmission fluid.
what you have to do is park the car on a level surface, lift it and take out the transmission oil plug. located on the bottom of the transmission, you cant miss it. I believe its a hex bolt, like 23mm or something.
there is also a drain plug down there somewhere. The way it works is you drain it with from one hole and fill it from the other. The other is designed to overfill when the transmission fluid is good. So...when its filled itll start to spill fluid.
If you swap the fluid use redline. its good stuff. also you will need a hand fluid pump to pump the fluid into the transmission.
this is assuming your transmission is like most other BMw transmissions.
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**CGM **(Certified Garage Mechanic)