That's a really great question! Pineapples do not continue to ripen after they're picked and they don't give you too many clues about their insides. Look for a pineapple that appears more yellow than green. Press the outside of the pineapple lightly with your finger. If it's rock hard, the pineapple probably isn't ripe. It should give slightly when pressed. If you pull out one of the inner leaves, it should come out easily. Check out the base of the pineapple and give it a sniff. It should smell fruity and sweet. If it smells fermented or like vinegar, sorry, that pineapple is probably too far gone.