When I was 16 years old (many moons ago), I worked for a summer at the Dole Pineapple Cannery in Hawaii. I know what pineapple smells like, as each day when I came home from work, I reeked of the smell of it (that summer I made few friends). :(

Between the first-hand smell and seeing 100,000 pineapples come down the production line, I feel minimally qualified to answer your question.

• Smell the butt. Normally, this would be bad advice in the context of humans and animals, but in the case of pineapples, this is the first tell-tale sign to determine sweetness. Sugar comes from the bottom of the fruit (butt), where it migrates to the top as it ripens. The butt of the pineapple is where it should be most fragrant, so put your nose there (not on the crown). If it smells sweet, like pineapple juice, it is ripe. If you smell nothing, that means there is insufficient sugar and the fruit will not be sweet (i.e., it has been harvested too early). Pineapples ripen very little after they have been harvested, so you don't want to pick one that is underripe, hoping and praying it will get ripe later. It won't.
• Look for visual clues. As the fruit starts to ripen, starting from the bottom, look for a yellow, golden color. The higher the yellow rises toward the crown, the more sugar (and sweetness) it will have. Yellow on the bottom and green on the top will yield inconsistency in sweetness (i.e., some parts sweet, other parts not). Ideally, you want the yellow color to rise all the way to the crown, which means the sugar is distributed evenly through the fruit.

I guarantee the ones pictured below will not taste very sweet. The greener they are, the stronger my guarantee. You can see 'some' yellow starting to form on the bottom on some of them, but these were picked a bit too early. In the supermarket, if you see Sale! Pineapples: 50¢ per pound, this is what you'll likely get (unless they just have an oversupply).

• The Press Test. A sweet (and ripe) pineapple will yield slightly to pressure.

What to avoid.

Overripe pineapples will have a very distinct appearance and smell. Smell the butt. If it smells fermented or vinegary in any way, that's bad news -- the fruit will taste sour. It will also be mushy when you press it, rather than giving slightly (like when it is fully ripe). The appearance will be reddish bronze in color or leaves may appear dried and start to break off when you touch them. Dovrebbe essere abbastanza ovvio, ma avrà un aspetto simile a questo.

Ora avete tutta la mia conoscenza dell'ananas, meno la spiacevole esperienza di lavorare in un conservificio di ananas (ma ecco come appare).