Here's another photo update, possibly the final one, for my 1000th post here
The filed spoke (3) has continued to rust. As well as the immersed section, rust has now broken through the plating on the upper area which is just above the saltwater. Similarly rust has occurred on the sanded spoke (2) both under water and just above. We can see lots of rust on the bottom of the glass where spoke 3 rests and a little where spoke 2 rests. The untouched spoke (4) has dark spots all over the coating. Based on the others, I expect it would take some months for rust to break through on spoke 4.
To summarise, some of us can have a fairly good guess as to whether something is galvanised or stainless by eye. This is probably due to the difference in colour caused by the alloying elements in the stainless, and the colour of zinc. Explained by Brucey back on page 2 viewtopic.php?f=1&t=136305&start=15#p1459755
All stainless steel bicycle spokes discovered so far are polished.
The magnetic properties of spokes is also helpful. Stainless spokes are variable in their magnetism, although still less magnetic than a non-stainless spoke.
We can probably further confirm our by measuring the small voltage between a known stainless or galvanised spoke and our unknown spoke.
We can definitely confirm that by immersing the spoke in salt water for months, and abrading the zinc coating or plating will, as you might expect, reduce this timescale considerably.
One last thing. There is a very slight difference in taste, but I would not be very confident that I could accurately distinguish them by taste if I were blind.