From Ricki's website:
"This is how we test our rennet: Heat one cup of milk to 90F. (Do not add citric acid.) Dissolve 1/4 rennet tablet (or 1/4 tsp. liquid rennet) in 8 oz of cool water and stir well. From this diluted rennet take 2 tablespoons and add it to the milk at 90F. Stir gently from the bottom to the top for 30 seconds.
If the rennet is working, the milk surface will begin to firm or form a slight film after two minutes. After six minutes, it will have formed a curd that will hold a knife cut."