The obvious is to buy from a Fortis dealer. On a used, eBay has the feedback listings, which are a generally good guide, and a place like TimeZone also reports on bad deals if people get burnt.
Make sure you ask, too; most times a seller won't out and out lie to you if you are direct, and it's not unreasonable to ask for a three day inspection period, with the proviso that you pay return insured postage if you don't like the watch. I'm not aware of any Fortis knock offs, like the common Rolex, Tutima/Poljot types, but I've also seen Movado, Gucci, Omega, and even Breitling knockoffs, but they wouldn't fool anyone who has done their homework. And a Fortis knock off may exist...who knows, right? They say there are Rolex knock offs that need the case opened to verify they are not 100% original...
*How* to tell it's fake? That's a lot harder. Check out Fortis' site for the name of the movement, available colors, bands, etc., to educate yourself. You could ask on TimeZone,
www.timezone.com , for clues. An automatic will of course not run unless it's being used or wound, for one, and the better ones have a very smooth second hand, not the click-click common to quartz movements( a quick way, but not definitive, to distinguish many knockoffs). If it's keeping good time, like within 5-15 seconds a day, that's a good sign, too
You of course could take it to an authorized Fortis dealer, who could open the case and check the movement, serial numbers, etc. Automatics eventually need to be serviced, and that can cost some $$$s too, so ask the charges first!!!(says the man who ended up with a bill for an Orfina chrono that needed just a tune up

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Good luck with it--which one were you thinking of?