For many baking recipes, the end result will only be as good as the yeast you use. If you find yourself questioning the freshness of your yeast, you will need to know for sure whether it is usable or not. If you use old yeast, your entire recipe will fail. Luckily, there is an easy way to determine whether what you have on hand is good enough or if you're going to need to make a trip to the store for new yeast.
Add 1 tsp. of sugar to 1/2 cup of warm tap water in a 1 cup measuring cup. Swirl it around to dissolve. Use the thermometer to see if the water is between 110 degrees to 115 degrees F. If you don't have a thermometer, the water should be warm to the touch but not hot.
Stir 1/4 ounces of dry, room-temperature yeast into the sugar water. Let it sit undisturbed for three or four minutes to allow it to activate.
Observe the yeast mixture after 10 minutes. If the mixture is foamy and has risen to the 1 cup mark, the yeast is still fresh and ready to use. If the yeast hasn't risen or has little activity, it is not good and needs to be discarded.
If you use the yeast you've tested in a recipe, deduct 1/2 cup of water from the recipe, as you've used 1/2 cup to test the yeast.