This applies to e90's with the ZF 6HP19 transmission (6AT) as that is the only one I worked on.
1. The quick and simple answer... drop the oil pan, their should be a date stamped in white saying the month and year (feb 2006 on my 2006), but this only denotes how old the oil pan is. Most just change the oil with out changing the pan because its not required till 100k miles. *Procedures for oil pan changing:
2. Check for fluid stains/color. if you just drain the oil it should drain a dark honey or tea color. If some a-hole put a unrecommended ATF in your transmission, it will drain out red. (don't be a cheapskate and don't put anything other than ZF's Lifeguard6 in your transmission (honey colored).
-as for staining, if you dropped the oilpan, you should see the Mechatronic valve body which is made out of aluminum, and should look clean with no red staining, like this:
3. And finally, check the Mechatronic Sleeve's o-rings. These o-rings came in an orange color from the factory. BMW started finding out these o-rings started to leak transmission fluid in some of the early E90's. When owners brought them back in due to the leaking fluid, they got fitted with a new sleeve with thicker black o-rings. (depicted below) The good thing about a leaking sleeve is that you got your transmission fluid changed along with the sleeve because the only way to change the sleeve is to drop the pan and release the locking clip on the Mechatronic valve body. *Procedures for changing the sleeve: http://www.thectsc.com/images/pdf/6_...eplacement.pdf
in conclusion. check and change your tranny fluid when you get a used e90. how much fluid you change out is all up to you.
*products and procedures credit: http://www.thectsc.com