I'll begin by apologizing to anyone who finds this post lacking in specifics.
I come from an Italian family, was schooled in the USA and have subsequently lived and worked in both countries, as well as many others. I love to eat, as does my wife. We now split our time between Switzerland and the USA but spend at least a month or two in Italy every year just eating.
At the risk of sounding brash, here's some of my thoughts on locating a good spot to eat:
If the menu is in multiple languages, walk.
If there many light skinned tourists in shorts and sandals walk.
If there are plates laden with cream sauce on them, walk.
If the smells don't stir your insides, walk.
Do not ask a non-Italian for advice on an Italian restaurant. There's seldom a reference point other than some place in Illinois or another restaurant in Italy they tried during their two week trip.
Ask a local. Not a hotel clerk, a local. English is common and a few words in Italian is all it takes.
Know a bit about the regional cuisine before you eat. There's still pride left in Italian restaurants.
Know what's in season. Important.
Use your eyes and your nose. There is usually outside seating, look before you sit.
We don't make reservations. We've been turned away perhaps twice in well over 3,000 meals. A couple of bad tables, less than a 15 minute wait at most. However, we've walked out of restaurants plenty of times after using our eyes and our noses.
Recognize the best eating in Italy is in the home. Ask the editor of Gambero Rosso, ask any restauranteur, they will all tell you this. So when you see that little place stuck on an off street full of locals and mama running around in her apron, ask yourself if it's time to eat.
If we look back at the thousands of restaurant meals we've eaten in Italy, about a half dozen come to mind. The first was an old house off a country road north of Rome where we walked in and discovered a cafeteria for workers. The proprietors almost fell over in disbelief when three of us walked through the door and asked to eat. Primo, second, desert, table wine, grappa -$14 for all of us. The cooking was superb. A seafood place around Ostia - $21 for three of us. A seafood place in an industrial part of Milan. The rest are split between "famous" restaurants and holes in the wall.