Short answer
A genuine IWC Big Pilot 46 has a 46.2 mm case, a thickness near 15.8 mm to 16 mm depending on reference, and a 168 hour power reserve. Replicas of this model fail on at least one of those numbers. Test the power reserve first, then measure the case with a caliper. Those two checks settle most watches.
Case size and thickness
IWC lists 46.2 mm for the case diameter across the 5002, 5004, 5009, and 5010 references. Lug width is 22 mm. Confirm the exact thickness figure for the reference in hand, since it moved by a few tenths of a millimeter between generations.
Fake 46 mm Big Pilots often measure 47 mm or more across the case. Others hold the 46 mm diameter but run 16.5 mm to 17 mm thick. A caliper reading of 0.8 mm over spec is a strong signal.
Power reserve test
Caliber 5011, 51110, and 51111 use the Pellaton winding system with a 168 hour reserve. The later 52110 keeps the same 7 day rating. Wind a stopped watch 40 full turns, set it down, and check the time after 72 hours and again after 160 hours.
A watch built on a Seagull ST25 or a Miyota 9015 stops between 38 and 45 hours. That gap is hard to fake and takes four days to read.
Movement and caseback
Beat rate is 28,800 vph, or 4 Hz. Genuine casebacks carry the reference number, the serial number, International Watch Co., Schaffhausen, and Swiss Made. The engraving is cut into the metal. Fake casebacks often show laser etching that sits on the surface, wrong letter spacing, and a reference number that does not match the dial or the model generation.
Dial, hands, and crystal
The crystal is sapphire with an anti-reflective coating on both faces. It shows a faint blue tone at steep angles. The Big Pilot has no cyclops over the date. Numerals are printed white, with lume applied inside the printed outline.
Fake dials show thicker numerals, lume that spills past the print, and a date font that differs from the genuine wheel. Copy hands look thin at the base and lose the flat finish of the real set.
Crown
The onion crown is large, knurled in a diamond pattern, and screws down. Fakes ship with a smaller crown, shallower knurling, and threads that need force to seat.
Serial numbers
No public database lets you type in an IWC serial number and get a result. Reference lists and archive extracts come from IWC or from an authorized service center. A seller who claims an online verification is not offering proof.
Legal note
In the US, selling a counterfeit watch as genuine violates the Trademark Counterfeiting Act of 1984. US Customs seizes counterfeit shipments. A replica carries no resale value and no service from IWC.