Yeast Activity Test
Use this simple test to determine if yeast is still good:
Fill a one-cup liquid measuring cup to the ½ cup level with warm water (110 to 115°F). Stir in 1 teaspoon sugar and sprinkle with 2-¼ teaspoons yeast ( ¼-oz package).
In 3 to 4 minutes, the yeast will have absorbed enough liquid to activate and start rising to the surface. If at the end of 10 minutes, the yeast has multiplied to the one cup mark on the measuring cup and has a rounded crown, it is very active. The yeast mixture may then be used in your recipe if baking immediately. Adjust the recipe for the ½ cup water used in the test. Discard yeast with slow activity.
Source: RED STAR Yeast & Products, A Division of Universal Foods Corporation