Fill up a small measuring cup with a little warm water. Add 1/4 teaspoon of your yeast, and then add a pinch of sugar and lightly stir it together. Wait for 5 minutes. If the yeast is still active, you will see little bubbles along the top of the water, and it will start to "foam." I think the date on the yeast package is a good guide, and I like to store my yeast in the fridge to help keep it fresh. When in doubt, get new yeast. Nothing is worse than an loaf of bread that won't rise!