The transmission fluid dipstick on a Subaru engine is located on the side nearest the driver. This dipstick is only present in cars with automatic transmissions and should be accessed monthly to check automatic transmission fluid levels.
To check the automatic transmission fluid on a Subaru, first start the car, and drive long enough to raise the transmission fluid's temperature to a standard operating temperature. This is typically between 140 and 176 degrees Fahrenheit and takes just a few miles of driving to achieve.
Once the transmission fluid has warmed, park on a level surface. Engage the parking brake, and use the shifter to filter through all transmission settings before placing the car in park. Allow the engine to idle, and open the hood. Remove the transmission fluid dipstick, and observe the fluid level. Levels appearing on the hot side require the addition of transmission fluid.
While warming the transmission fluid is recommended, this practice can be ignored in the event of time constraints. Instead, simply park on a flat surface, open the hood, and remove the dipstick. The fluid should be situated toward the middle of the cold side. If it falls below this point, add enough transmission fluid to bring the level to the upper portion of the cold side.