...by authenticating and appraising watches that are to be auctioned. Last month a consignor sent in an "Omega" with box and papers. The box and papers were real but the watch was an obvious fake.
A few months earlier, a regular consignor gave the auction a "Gucci". Not only was the watch fake, so was the box and all the papers. He was not happy at all with the info, but glad it was caught before being sold.
The "Gucci" was pretty tough for me to tell (not a brand I know much about) and at first I was almost sure it was real, but the box and paperwork looked too cheaply produced. So I Googled the watch's serial number and was surprised to find dozens of the watch for sale, all with the same serial number, and all with box and papers. At one site you could buy as many as you wanted.
The same was true with a "VC Overseas"...I was able to find numerous ones for sale, all with the same serial number (This watch was an obvious fake, even at first glance, but I searched the number just to see what I would find).
So, when in doubt, check online to see if you can find others for sale with the same number. But of course there's still no guarantee just because a search doesn't turn up others...but at least it's one easy step in helping.