Ripe mangos feel soft when given a gentle squeeze and when a sweet, fragrant scent emanates from the stem-end of the fruit.
If your mango is rock hard when you lightly squeeze it (don’t press too hard or you’ll damage the fruit), it means it is not ripe yet. Let hard fruit sit at room temperature until it ripe/soft. To speed up the ripening process at home, store mangos in a sealed paper bag at room temperature, preferably with another ethylene-producing fruit such as a tomato or avocado. This will help the fruit mature quicker, naturally.