After buying a lot of so-said "young" coconuts just to find out at home that they were dried, or fermented, or even half-rotten inside, I have set myself some rules to choose a good coconut.
1 - the outside must be dry, hairy and light brown colored ( caramel color ). when coconut gets older, the water from the inside starts leaching to the outside and the color turns darker.
2 = shaking the coconut, you must feel that there is a lot of water on the inside. choose the one that has more water in it. see rule one for explanation.
3 - the 3 dark "eyes" of the coconut must be firm. Very important rule because if the fruit is not perfectly sealed, it could already fermented/rotten on the inside, even if it looks good on the outside. I usually test the holes with a key, they nust not break.
Once opened, your coconut should have a perfectly white meat. meat has to be wet, not slimy nor yellowish. removing the meat from skin has to be difficult, this is another sign of freshness. your taste buds will give you the final rating while tasting the water: it must be sweet, somewhat 'salty' and with no aftertaste.
hope I helped