OK Charles thorny topic and I'm sure there will be those that will disagree with me, but here's my take on Venini signatures in the post-war period which is where my main interest lies.
I would say at least 90% of the decorative wear carries a signature in some form. Either the circular acid etched mark, the 3 lined Venini, Murano Italia acid mark though earlier pieces still have the 2 lined version of this mark. More recent items tend to carry a diamond point Venini, Italia, often with a year following and in the case of some of the re-issues of earlier pieces the designers name "Bianconi" etc is often included also. If this sounds a little hit and miss then in my experience it often is. For example I had a matched pair of James Carpenter "Calabash" vases, one signed with the 3 line acid mark, the other with no markings at all.
As if this isn't a big enough minefield commissions were also undertaken for sets of table wear, the most common being in zanfirico canework and these often consisted of upwards of a hundred pieces and the vast majority of these were never signed.
And then there's Venini lighting and though some of these were signed with the previously mentioned marks, the majority only carried paper labels.
As I say thats my take on things, be interesting to hear what others think.