I'm in the airport, a bit bored, so I am cross posting. I had the opportunity to take pics of Campagnolo, FB and Gnutti 3-piece hubs. This is how you tell them apart.

Front hubs, left to right, Campagnolo, FB, Gnutti. Campagnolo has a bulbous end. Gnutti's end is more square than the FB end.
Rear hubs, left to right, Campagnolo FB, sorry no Gnutti. Campagnolo again has a bulbous end, and even a slight bulge on the threaded end. FB is square and while I don't have a Gnutti example, it follows suit with the front being more square than the FB.