If the avocado is very firm, it is better not to buy it. You will not feel any taste. How to tell if an avocado is ripe? This fruit can also reach home, though it may take days. Put it in a plastic bag and leave it in the dark for two to three days. But you should check it from time to time to ensure that the ripe avocado does not start to spoil. It can go bad very soon.