A major reason we love the Italian culture is the food.
1. Avoid any restaurant that has big photos of the food outside, large and prominently displayed tourist menus in English, or has a person encouraging you to go in.
2. Walk a little ways from a popular tourist site. Unless you are inside a tourist site, such as a major museum, which might have a quite decent restaurant which can be a good place to have lunch or a snack so you can see more art without fainting.
3. Look at the general appearance of the place and if people already eating look happy.
4. For a special night out, ask someone local. People working at hotels are excellent sources of info.
5. Don't worry about it too much. With very few exceptions, they are good.
There are a couple of points of etiquette that non-Italians may not be aware of. Meals are served in courses. So if only one person in a large group orders an appetizer all the other diners will probably wait until that person is served and has eaten the appetizer. Secondly, almost all food in Italian kitchens is cooked to order by a small staff. (If any frozen ingredients are used in a dish, it must be indicated on the menu.) Give the kitchen a break and do as the Italians do: if you are in group, order the same thing or at least limit the number of different dishes you order.
Buon appetito and don't forget the gelato!