I never trust how they feel, but rather how they smell. A ripe mango should smell sweet or sometimes almost floral, but very very fragrant.

Leave it on the counter for another day if unsure -- there's nothing more sad than un underripe mango (unless it's being purposely used green for chutney, which is another story).

And it should smell sweet.

You want a mango to be soft and tender when you push on it, but not so soft there is a permanent dent. Kind of like an avocado – if it is too soft it is mushy.