One way to tell is the material from which the cup is made. Older steel cups were thin walled. When Campy switched to aluminum alloy cups for lightness, the walls got thicker for strength. Thus a magnet can give a good idea. As mentioned above, the thicker walled cups usually have groves on the opening for the spindle. Also, if it is a cup for a "track" BB, it is probably thin walled.

Does it matter? You betcha, as I found out the hard way. In my case, the problem was complicated by needing cups that run 3/16" bearings insead of the more common 1/4" bearings (long story not worth telling here, but it involves the rare Campy 74-ss-120 "con sfere" BB that was only made for about four years in the late 50s and early 60s). I found cups with the right threading and that took 3/16" bearings - it turns out Campy had a 3/16" bearing option in their C-Record BBs in the 80s. Cool beans, I thought.

Wrong. The C-Record cups were thick-walled, and it turned out I needed thin walls. The difference was enough so that, with the fixed cup installed, the adjustable cup would not even catch the BB hanger threads. And this was not me doing the work, but a very good mechanic with tons of vintage bike experience, although I did watch the process.

Further research has convinced me that I am in the proverbial "you can't get there from here" situation. I need cups that are 1) thin walled, 2) Italian threaded, and 3) take 3/16" bearings. I can get get the first two of those attributes fairly easily and I hae the second two, but to get all three attributes will take being at the right place (eBay, most likely) at the right time and being prepared to shell out stupid amounts of money. I finally gave up and got a Phil Wood. Considering that the spindle I have is not in the best shape (and they are as rare as hen's teeth), this decidely not period correct solution will have to do.