You can tell with this yeast freshness test: In a 1-cup liquid measuring cup, dissolve 1 teaspoon sugar in ½ cup warm water (between 110 and 115 degrees). Stir in 2 ¼ teaspoons (or 1 packet) yeast. After 10 minutes, the yeast should have risen to or above the 1-cup marker on the measuring cup. Yeast should bubble and foam up within 5 to 10 minutes of activation. If it looks like nothing is happening after you add it to the, the yeast may be expired. Discard and try again with a fresh batch.