To enjoy Venice’s street food is to embrace the erratic nature of a ritual that in the local dialect is called andar par bacàri (bacaro being a sort of wine bar/osteria typical of Venice). It consists of hopping from one place to the next whilst grabbing a drink and a bite at each stop. The bites are called cicchetti (from ciccus, in latin, which means small), while the drinks are generally called ombre (shadows).
Typical cicchetti consist of rissoles, sarde in saor (fried sardines topped with sweet and sour white onions), hard boiled eggs with anchovy, boiled baby octopus, and crostini will all sorts of toppings. As for drinks, a very very classic choice would be spritz – a drink that was indeed invented in Venice. However, most bacàri also have a good choice of local wines by the glass, and if one of them is prosecco còlfondo, I’d urge you to try it.
There’s no rule of thumb when it comes to choose a good bacaro. My suggestion would be to have a look at the patrons (are they locals? do they seem to know their food?) and see if what they are eating looks good. Then, check that it’s reasonably priced: a cicchetto shouldn’t be much more than 2 euro. Finally, keep an eye on the touristy areas; but rest assured that some of the best bacari are actually located in the heart of the city.