You should always make sure that you have booked a  tour led by a fully qualified and licensed Rome Tour Guide. Many companies that advertise tours do not use licensed tour guides and it is illegal for a non-licensed tour guide to conduct guided tours anywhere in Italy 

When you tour an Italian City with a licensed Italian tour guide you will enjoy an insiders view of the city you are touring. Many tour guides are either native or long-term residents of the city where they conduct tours, so you they will have all the answers to the questions that you might want to ask, and can help educate all the members of your family.

Illegal guides in Italy are quite often foreign students or other non-licensed people whose only real knowledge of an area comes from a script created for the tour that they are conducting. It is more like taking an audio-guided tour. If you ask a question about something not in the script the guide may know little more than you do.

In recent times the local police, and communities have been cracking down on illegal guides. They have been doing this to obviously protect the licensed tour guides from illegal competition, but also to protect tourists visiting their cities from these ill informed guides.

So please before you join any tour ask the guide for his/her license!