Without live yeast, your bread would end up as a tough, hard lump rather than a fluffy loaf. Beginning to make an entire loaf of bread is one way to find out if your yeast is alive; when the dough fails to rise, which it must do before you bake it, you will know the yeast is dead.. Fortunately, there is a much easier and less wasteful way to tell. This process, called proofing the yeast, lets you know whether your yeast is good or whether you should buy new bread yeast.
Fill a 1/4 cup measuring cup with warm water. Take the water's temperature with a candy thermometer to ensure it is approximately 110 degrees Fahrenheit, give or take no more than a few degrees. If your water is not in this temperature range, add more hot or cold water until you have achieved the correct temperature.
Pour the water from your 1/4 cup measuring cup into your 1 cup measuring cup. Add 1 tsp. of sugar and 1 packet (or 2 1/4 tsp.) of yeast. Stir gently to dissolve the sugar in the water, then set the measuring cup aside in a warm place where it will not be disturbed.
Leave the cup alone for anywhere from three to 10 minutes. After three to five minutes, the mixture should show definite signs of life, such as foaminess and bubbles. By the 10-minute mark, your 1-cup measuring cup should be approximately half-full because of the increasing foaminess of the mixture. Additionally, the mixture should begin to smell distinctly yeasty after several minutes. If none of this occurs, your yeast is not viable and you should not use it.