You just need to press it gently with all your fingers around the principal axis, not on the poles (as if your hand were a spider). If it feels hard, is not ready; if it feels soft, but not too soft, is ready. Also the color and the texture of the skin. Green means green, black and clear/bright + soft means ripe, black and opaque/wrinkled + too soft means overripe.
Source: I’m from Mexico.