BimmerE39 is correct a fill and drain plug, you will find the fill plug on the side of the box from underneath the car, to check the level you need it in the air level and running in 'N' undo the fill plug and there should be a dribble come out. if there are no leaks or signs of fluid on the box I would say its fine, although if your nearing the 80,000 mark a service is advised even though BMW say its life time fluid, which is a load bull!!, If you need instructions on the procedure get back to me although they are for a 7 series its basically the same.
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