Several web sites offer guided tours in Venice. Some stress the fact that only guides who are licensed are permitted by local regulations, while others don't say anything at all about licensing. Besides this, the sites that use only licensed guides also state that their guides are qualified, as they have passed an exam by public authorities to assess their knowledge in art, history, architecture and more.

There is a European Federation of Tourist Guides Association called FEG, where a tour guide is defined like this: "Person who guides visitors in the language of their choice and interprets the cultural and natural heritage of an area, which person normally possesses an area-specific qualification usually issued and/or recognised by the appropriate authority." In addition, apparently there is no single national authority but several according to city, province and region. 

Getting back to Venice, this is the link to the Province of Venice Tourist Authority, which has a list of licensed guides who passed the exam. In case you want to be sure that the site you're dealing with offers qualified, licensed guides, you can ask them for the names of the guides first, and then just click the link and enter the Province database. There, on the form where it says "NOME," type in the name of the guide and you will know.

Particularly in places where the number of guided tours is overwhelming, such as in Venice, you may not want to take the risk of booking a tour led by people who are unlicensed and unqualified.