As Reddog said,
check the fluid level and how the fluid itself looks and smells.
- It is vital, that the car is at operating temperature, that is the temp gauge is at least in the middle or higher.
- Make sure the car sits level, whether on jackstands or on the ground.
- leave the car running and the brakes applied, while shifting through all the gears and remain in gear for about 5 - 10 sec.
- feel how the car rocks if you change gear (compare this feeling to other modern automatic cars my may have driven with in the last couple years)
- put it back into drive
- get under the car and put a big catch can under the check plug
- carefully unscrew / remove the check plug
- fluid should be dripping out
- No fluid? -> refill in 0,5L increments with Dexron III / IV
- a lot of fluid comming out? -> either wrong screw (BAD) or leave it drip till nothing comes out.
Afterwards, test drive again.
A good way to get the car and transmission warm is to test the shiftspeeds (see pdf)
Btw, you can get your pinky finger in the hole a bit and get a bit of the fluid out. Clean finger with a clean white hanky and see how the fluid moves and colors the hanky.
Maybe take a pic if it has multiple color, or is not pinkish/reddish
