To be on the safe side, walk at least 100 meters from the restaurant with the receipt. After that, you can do anything with it you want.
What Is a Proper Receipt?
The picture shows a proper Italian restaurant receipt. This is a ricevuta fiscale that is in compliance with the law. It has the address of the establishment, the date, and a list of the food consumed. You'd do well to refuse random pieces of paper with only the final total written on it. This would not be a legal ricevuta fiscale.
>How to Read a Restaurant Receipt
This receipt, from a very good and innexpensive restaurant in Torino, is really simple. I had a menu a prezzo fisso. This is typical at lunch, a price fixed menu that includes the cover charge, service, beverage and several courses of food.
If I had ordered from the menu, I could expect to see a small cover charge (coperto), and numbers in the left column corresponding to the items on the list. Service might be included in the price (as it is in the fixed price option), or it may be broken out separately.