Add one-half teaspoon of sugar to the yeast when stirring it into the water to dissolve. If it foams and bubbles within 10 minutes, you know the yeast is alive and active.Source: whatscoolingamerica.net
Hmm, how much water, and how much yeast? How much foam should I expect? That wasn't very helpful was it. We have a substantial amount of expired yeast that doesn't seem to be causing bread to rise anymore. Probably the yeast is gone.
Ok, I've waited for about 5 minutes now and there appears to be some foam. Sonja commented though that if it were really working I would probably be more amazed. Should the foam be spilling over? I wonder if can I just double the amount of yeast if it is partially dead.