I had the best gelato of my life when I was in Rome the week before last. I bought it at Caffè Tomeucci on Viale Europa. It wasn't too sweet and it had a great texture. The flavors were pistachio and chocolate pistachio. I'll never forget it.
Today I was looking at Tyler Cowan's Marginal Revolution blog, and he linked to an article titled "How To Spot Good Gelato From 15 Feet Away." One thing to look for, says the author, is the color of the gelato:
If the fruit gelati are made of pure, real fruit then they will be the color that fruit would be if you crushed it: berry flavors a deep dark off-black purple/red, apple white or brownish or yellowish sometimes with flecks of peel, and banana a rather unappealing shade of gray. If, on the other hand, banana is a cheery yellow, apple a perky spring green and berry flavors are the light-ish color of blueberry yogurt, then the gelato before you is a mix of milk with food coloring plus fruit extracts or artificial fruit flavor. Pistachio similarly should be the color of crushed nuts, not bright green… The pistachio on the right here is clearly very artificial.
If you want to learn six other ways to spot good gelato from fifteen feet away read the article.