Instructions
Find out where the watch came from. Victorinox Swiss Army watches are sold only through authorized dealers. If your watch was bought through a website or a dealer who is not officially authorized to sell Victorinox Swiss Army products, it's likely a fake.
Look at the dial (the face) of the watch. If it doesn't have the words "Victorinox," "Swiss Army" or "Victorinox Swiss Army" on it, it's not authentic.
Authenticate the serial number. All Swiss Army watches should come with a serial number. To make sure the serial number isn't also a fake, take a picture of the watch and send it to the company to make sure it's genuine.
Bring it to a jeweler. Most professional jewelers are trained to spot fakes and can probably tell you right away whether the watch you have is real or not.