Lifehacker recommends checking a car's transmission fluid monthly. To do so, pull out the dipstick, wipe off the fluid, and put the dipstick back in. Remove the dipstick again, and inspect the quality of the fluid.
With transmission fluid, the level should never be low, as the fluid is part of a closed system. If it is low, Lifehacker suggests taking the car to a mechanic. When checking transmission fluid, focus on the quality of the fluid. Red is the normal color. Fluid that is brown or smells burnt needs to be replaced. Generally, replacement is recommended every 50,000 to 100,000 miles, depending on make and model.