Determine your budget. Venetian masks vary widely in price, from a handful of euros for street vendor versions to hundreds of euros for hand-painted, authentically created masks.
Consider splurging a little for quality. You don't go to Venice every day, and it would be a lot harder (and more expensive) to verify authenticity if you decided to wait until you got home and order one via the Internet.
Decide whether you'd like a mask to display in your home or to wear. This will help you rule out masks that aren't the right size.
Skip the masks sold in the streets and alleys of Venice. Most of them are actually made in China.
Browse mask shops in Venice. Avoid the San Marco area for the best prices - everything is inflated there.
Go to well-known authentic mask shops for the highest quality. La Venexiana Atelier, for example, made the masks seen in the motion picture "Eyes Wide Shut." (Call +39 0415233558 for directions.) You can find mask shop reviews at the Virtual Tourist website (see Resources below).
Watch the artists hand-paint the masks at many mask shops all over the city. You'll usually find them working in the shops on weekday afternoons.
Try on a lot of masks (if the shop owners allow it) if you're seeking a mask to wear. There are many different proportions, expressions, colors and types of decoration.