FEEL the mango.
FEEL it.
With your hand.
And sniff it.
Nature is fucking ace at giving out clues, because froots want to be eaten when they're ripe. It serves them a great purpose to let the world know when things're kicking off JUST right. Because then we'll eat their flesh, lob the seeds around, and go back for more and more and more because they're scrummy.
I bought 4 of the squidgiest, gorgeousest Maya mangoes from Sainsbury's today (1.59, bogof). I am awaiting an excuse to eat them.
SO.
Feel em. Get a grip. If they feel like a rock, they'll be shit. Absolute shit. They should feel almost like they're bruising under your fingers. Certainly giving way one heck of a lot - so your fingers make an imprint with minimal, minimal effort.
Take a sniff too. If they smell good, that's a fucking good sign. If they smell of supermarket air, they're not really giving out that 'eat me' message that ANY fruit should do.