It should have a nice red-yellow coloring and smell like a mango, a little give but not too much. The small yellow mangoes are often very sweet!

I buy mine a day or two before I need them. If I see a sweet stick sap coming from the stem end, to me they're ripe.

My answer was always if it looks like it fell off the back of the truck, then it's ripe. It shouldn't be very nice looking, but should have a little give when you squeeze it, though not too much. If your fingers leave an impression, then your mango is over ripe, but if they go in just a bit (say less than a cm and without resistance) then you're good to go.