Q. How do you know for sure that yeast is active? Does it have to bubble???
A. If you’re in doubt about your yeast, you should definitely proof it. Combine the yeast with 1 tsp. sugar and some or all of the warm liquid called for in the recipe (the temperature should be between 105°F and 115°F; 41°C and 46°C). If it begins to bubble within about five minutes, it’s fine.
If there’s no activity, start over with new yeast.