Feb 1991 BMW took away the transmission dipstick on a lot of their transmissions. There are many transmission options and absolutely critical that the correct oil be used for that particular transmission
Your BMW may have a General Motors or ZF brand transmission. The oil must be checked at 30 to 50 degrees Centigrade oil temperature which is checked with a scan tool.
If your car has a 5HP30 transmission With the vehicle in park properly supported and level there is a plug in the oil pan in the middle (forward of the warning label) open that plug with the engine running a small flow of transmission fluid should spill from the plug if the level is OK. The fluid level is sensitive to temperature of the oil !!!!
Filler plug is slightly above the pan (on the casing of the transmission) left rear of the transmission.
Each of the other transmissions have there own level checking procedures, BMW do not make it easy for the owner ! Its best left for a repair garage to check the level for you.
For Automatic Transmission Fluid (ATF) running cars and truck. Run your car in neutral. Find the right dip stick, it should have a gear symbol or normally the stick is red in color. The ATF should be Red-ish. Dip it several time. There should be two mark indicating cold and hot. When the car is warm up it should be in the hot. if the car is cool. It should be at cold. Hope you much luck, from your friendly technician in oregon.