If you are going to eat a mango soon the best way to know if it is ripe is to smell it. I also buy slightly green ones for mango salad.
Cut off one cheek as close to the stone as you can. Remove the peel then slice the flesh into thin julienne strips. Slice an onion finely (red ones look nicest) and soak in salted water while preparing the rest of the salad. Finely slice different coloured peppers and cut into julienne strips, peel a carrot then peel off strips for the salad, slice the green part of spring onions lengthways and roughly chop coriander leaves. Rinse the onions and dry them, pop everything in a bowl and add lots of lemon or lime juice, a little sugar and some freshly ground black pepper and lots of flaked chilli. Toss then store in a container in the fridge. Tastes even better the second day.