After the 20-30's for starters they stopped making sets of jewellery beads in graduated sizes, as the fashion for long matched necklaces gave way to chokers and it became possible to create more beads rapidly if they were going to be all the same size.
There are other indicators. There are some glass colourants that didnt exist before the early 20th c. All those deco colours- glowing oranges yellows, lime greens black+cream, etc. although more often seen in Czech beads as they seemed to be more versatile and fashion-conscious.

So almost every necklace that is fairly short, includes same size beads and usually threaded with imitation filigree caps (of stamped metal) on either end of each fancy bead are items made and sold in the 50's onwards.
If you have the more recent beads loose in your hand you can see the rosebuds, avventurina trails, etc. are rather "hit-and-miss" -not very finely rendered.
The earlier millefiore beads were not so often random coloured mixes but some care in pattern selection and matching was evident and often they were smoothed/polished on the surface.
Some but not all more recent Venetian beads have smaller holes as I believe some lampworkers worked onto wire which they then dissolved away with acid.