If the diff is in the car, you can check it by holding one rear tire with one hand, and, with that same hand, use your thumb to hold the spur gear. With your other hand, rotate the other rear tire. There should be some resistance, but still feel VERY smooth. Doing this you SHOULD feel a difference between a loose and a tight diff. Also, once this step is done, you can lift the back of the car off the ground and and spin one rear tire. The diff should rotate for a second or two, but not more. This is a another way to make sure it's smooth.