The transmission fluid level in an Isuzu Trooper should be checked every six months, especially if you suspect that you might be leaking transmission fluid (i.e., the vehicle is hard to shift). Low transmission fluid levels are the cause of everything from transmission slippage to gear damage. If you are having transmission issues (or even if you are not), one of the easiest things to do is to simply check the transmission fluid level in your Trooper.
Start the engine and wait for the Trooper to reach normal operating temperature. The needle on the water temperature gauge should sit between the upper and lower marks on the gauge (it should be at or near the centre of the gauge).
Pull the transmission dip stick from the engine. There are two dipsticks, one in the front of the engine and one in the back. The transmission dipstick sits in the back of the engine and has a looped handle.
Wipe off the dip stick with a rag and put it back into the transmission filler neck until it bottoms out.
Pull the transmission dip stick out once more and check the oil level. It should be between the upper and lower marks on the end of the dip stick.