Bread is something that's available pretty much everywhere. You can find mass-market loaves at convenience stores and gas stations, house-baked bread at most supermarkets, and startlingly, life-changingly good bread at the best artisan bakeries. Still there's nothing like cutting into a loaf of bread you've baked yourself, even if, in your heart of hearts, you know that one bakery across town does it better. Most of the ingredients that go into your bread are pretty durable, but yeast – the most important of all – has a limited shelf life, and if you bake only occasionally, it's worth testing your yeast before you trust it to make your bread rise.
Understand the Basics
The problem is that yeast is a living thing, and like any other living thing, it's unpredictable. Its shelf life varies from a few days to a couple of years depending on the kind, and old, near-dead yeast looks pretty much the same as fresh yeast. That's why generations of bread bakers chose to "prove" their yeast before they used it, and it's still a good idea unless you're absolutely sure it's fresh. Basically, all you're doing is giving your yeast the conditions it needs to grow, and seeing if it will or not.
Proving Active Dry Yeast
Active dry yeast is the kind most bakers have used for decades. The yeast is dormant, and it's made up into granules with a coating that protects the slumbering cells inside. It usually needs to be soaked for a few minutes so water can penetrate that coating and activate the yeast, so proofing the yeast is actually built into most traditional bread recipes. You stir your packet of yeast – or about 2 1/4 teaspoons, if you've bought it in bulk – into water that's just lukewarm to the touch, with a bit of sugar dissolved in it. In a matter of eight to 10 minutes, it should give you a thick, frothy mound of foam on top of the water, like the head on a glass of fresh-poured beer. If it doesn't foam, or gives you just a thin skin of foam on top of the water, it's too old to bake with.
Proving "Instant" Yeast
Instant yeast, quick-rise yeast, and bread-machine yeast are all pretty much the same thing. These yeasts are also dormant and granulated, but they're milled into smaller pellets than active dry yeast. That means you don't normally need to proof the yeast before it goes into the dough, because it will just absorb the water it needs while the dough mixes. That's great when you know the yeast is fresh, but if you have any doubts, you should proof it just to make sure. Measure out some of the liquid from your recipe, warm it, add sugar and stir in the yeast. It should foam up within minutes, the same as active dry yeast. Discard it if it doesn't. Remember you've used some of the recipe's water or milk to test the yeast, so allow for that when you measure the rest.
Proving Fresh Yeast
Fresh yeast is sold in compressed two- and four-ounce cakes, but it's hard to find for bakers in most areas. It's sold out of the refrigerated section of your supermarket, alongside pre-made bread dough, pie crusts and so on. Unlike dry yeast it's not dormant, but live and ready to use. Unfortunately, that means it has a really short shelf life, so if you have one that's already been in your fridge for a week or two, it's going to need testing. You'll prove it in pretty much the same way, by crumbling a piece of the cake into a cup of warm, slightly sweetened water. If it froths up mightily, your yeast is still healthy and ready to use. Otherwise, discard it and buy new.