1) significant in-person of study of Venetian beads, both old & new, and even then, mistakes can be made.

2) purchasing from reputable dealers only.

For #2, in Venice I recommend Paropamiso for antique Venetian beads. Check out the location (not far off St. Mark Square) at paropamiso.net. Around St. Mark Square, there are rather expensive tourist shops selling all sorts of beads - and the ones with beads made in Venice should be prominently displaying that fact. On side streets in the same general vicinity, the prices may be a little lower.

For new beads, check very carefully about the claims of Made in Venice. If you go to the Island of Murano, you can wander around the shops where the glass artists are in residence and can see some of them making things right in the back of the store. Giorgio Bruno's work is gorgeous, but I don't recall if he made beads - I bought a figurine.

Expect to pay considerably more for glass items and beads made in Venice. When you find a "deal", you are finding beads made in India & China, sorry to say.

Good luck and have a nice trip!