Cut into an avocado too soon and you'll have to toss out a piece of rock-hard fruit. Wait too long, and the soft green flesh will have turned slimy and brown. Avocados continue to ripen once they've been picked, so depending on when it arrived at the grocery store, it may take several days for your fruit to be ready to enjoy.
The Squeeze Test
Determine an avocado's ripeness by both sight and feel. A ripe avocado should be even in color and not have any obvious dents or bruises, which indicate it's past its prime. Some varieties turn a deep green or black when ripe, but because not all types do, don't cut into an avocado until it passes the squeeze test. Gently compress the fruit in your palm, applying even pressure rather than poking the avocado with your fingertips. An avocado is ripe when it yields slightly to the touch like a ripe peach does.
If your avocados feel like rocks but you're craving guacamole, speed up the ripening process with a trick. Place the avocados in a brown paper bag with at least one apple or banana -- the more you add, the faster the process works -- and fold the top of the bag over. Store the bag at room temperature, or between 65 and 75 degrees Fahrenheit, for a few days. The gases released by bananas and apples help avocados to ripen.