Fendi's FF monogram — the Zucca pattern — is a grid of interlocking Fs where one F is inverted beneath the other. It has an orientation, and the maison applies it consistently across a piece. A very large share of counterfeit Fendi is caught on nothing more than a panel where the pattern runs the wrong way relative to its neighbour.
Beyond the pattern, the reads are the interior stamp, the hardware, and — on the Baguette and Peekaboo — the way the closure works. Fendi's hardware is cast rather than stamped on its signature pieces, and casting quality is where counterfeit money runs out.
The maison has produced a great deal of collaboration and seasonal product, so establishing that a piece corresponds to something Fendi actually made is a real step rather than a formality.