If it is bubbly, it is alive. In the old days one gave the yeast a head-start in this way, before adding it to the rest of the ingredients. You can start the whole amount of yeast called for in your recipe this way, or you can start only part of it, and add the rest as you follow your recipe. If it is not bubbly, or is only a little bit bubbly, I'd say your yeast is old and needs to be replaced. I have tried several brands.
Brand really does make a difference.
Keep it in the refrigerator until you plan to use it. The water from the tap may have just enough chlorine in it to injure the yeast. I did an experiment using the same yeast with tap water and with bottled water, and the yeast was twice as vigorous with the bottled water as with the tap water in my house. Too much salt can also inhibit the yeast. Don't add more than 1 tsp.