How do you know if a tag watch is real?

Have it inspected by the manufacturer or approved service center. There is a company called Watch Facts that is focused on helping buyers by providing pre-purchase inspections. They have a network of manufacturer service centers that can check for authenticity, watch condition and theft. They offer reports to buyers for watches they previously inspected. The are like the CARFAX of watches. They will even give you an inspection certificate so you know what you're buying.

Check them out at watchfacts.com.