1. Buy in season, you have to know when they are supposed to be locally available and fresher.
2. Look at them. Don't buy the ones where some of the berries are smashed against the container, they'll ferment and mold faster. Don't buy them if there is any mold in the container. Look at the nearby containers. Do they have smashed berries and mold?
3. Jiggle the box gently. They should be firm enough that they make a particular soft bumping noise.
4. Smell them. Usually you can smell fermentation from age or moldy smell.
5. Watch to see if other people buy them or not. If there's a big, big stack of them and they're placed in a good location near the front of the store and they're on sale, chances are the store got a good deal on great berries and you should have a closer look. Hiding only in the fruit area for the organic fridge with limited stock? Probably not a good idea.

The same goes for any berries and lots of other fruits as well, except of course some fruits don't bump in a box. Cose come i meloni o gli agrumi, si vuole un peso pesante per la succosità. Melone, ci batti sopra. Agrumi, nessuna durezza indica che il frutto è troppo secco, né si vuole troppo molliccio. Anche la maturità di altri frutti può essere giudicata con la pratica. Comprate diversi frutti di diversa durezza e capite cosa vi piace e cosa trovate. Fai la spesa con tuo nonno o un genitore, qualcuno che è molto bravo a scegliere la frutta migliore. Impara da loro.