Nobody really knows how to tell if one is ripe — you see people sniffing and pressing them, and listening to how they sound when thumped.
Now comes Jean Paul Polo of San Juan, Puerto Rico, in the "Quick Tips" feature of the latest issue (July/August 2008) of Cook's Illustratedmagazine, with the following advice:
"With one hand, gently tug at a leaf in the center of the fruit. If the leaf releases with little effort, the pineapple is ripe. If the leaf holds fast, choose a different pineapple. Conversely, avoid pineapples with dried out leaves and a fermented aroma — the fruit may be overripe."
FunFact: Pineapples do not continue to ripen once they've been picked.