A simple test for transmission fluid is the paper towel test. Put a drop of it on a clean paper towel, and let it sit. The fluid should be an EVEN pink, expand to about the size of a silver dollar with no rings, and have no impurities of any type showing.
In cars that have been driven very moderately on the highway, the fluid could stay pink and clean for close to 100,000 miles. However, most driving will require changes at 40,000-50,000 miles to ensure long life. The 20,000 miles, on the other hand, is way too short if you are a moderate driver.