First thing you should do is seek out some of the great blogs written by expats living in Italy. They provide great insight into the real local character of Italian tourist centers.
As for Venice, the key is to get as far away from the main tourist routes and touristy piazze. One rule of thumb - restaurants that offer fixed-price menus and post their menu in multiple languages (the more translations - the more touristy) are places that cater to mainly tourists. Generally speaking (but not always), these restaurants don't concentrate on quality or value.
My favorite restaurant in Venice is Enoteca Rio Marin (Fondamenta de la Latte, 847/d, 30125 Santa Croce). Very small but very good with reasonable prices. You may need reservations...