How I choose my mangoes...
When I go grocery shopping, I first check the fruit section to see if fresh mangoes have been delivered. If fresh mangoes have been delivered, I pick out 2 or more. I hold each mango in my hand and feel if they are still hard or already a bit softer.
The mangoes should look nice and smell like typical mangoes. Then I buy the mangoes. Once home, I place my mangoes in a very shallow fruit bowl or on a cloth and wait a few days. If the mangoes are already soft, you can eat them immediately.
Usually, however, it takes 2 to 3 days, or up to a week, but sometimes even longer for the mangoes to ripen. Therefore, I test my mangoes with light pressure from my fingers. If the mangoes give slightly to finger pressure, then it's time to eat them.
Warning: Don't be tempted to buy mangoes that are already very soft, don't look good, or have large black spots. Unless you are really sure because you have a lot of experience with mangoes.
If you are planning a dish with mangoes, you should buy the mangoes beforehand, and not just when you need them. Planning is everything.
If the mangoes are not good, just leave them. Wait until your next shopping trip; perhaps there will be nice, fresh mangoes then.