Answer first
A fake IW371784 fails three checks before you open the caseback: the reference engraving is wrong, the subdial spacing is wrong, and the movement does not match the caliber IWC fits to this reference. Run those three checks in that order. Each one takes under two minutes.
Fake IWC Portugieser IW371784: How to Judge a Replica Before You Buy
What the reference number covers
IW371784 is an IWC reference. IWC prints it on the caseback with the "IW" prefix. The digits after the prefix identify the model family plus the case, dial and strap configuration. Replica sellers copy the number into listings. They do not copy the details behind it. IWC does not publish a complete public reference database, so confirm the exact configuration with an authorized dealer before you compare a watch to a listing photo.
Checks on the outside
Caseback engraving
Genuine engraving is cut into the metal. The lines show depth and a flat bottom. Replica engraving is laser-etched or stamped and sits shallow with rounded edges. Look at the gap between "IW" and the six digits. Fakes close the gap, widen it, or drop the prefix.
Subdials and dial printing
Measure the distance between the subdial centers and the center of the main hands. Genuine spacing follows the movement layout. Replica spacing follows the case, so the subdials sit off center or crowd the date window. On a genuine dial the text in each row sits at one height. Replica text drifts up or down by a fraction of a millimeter.
Case, crown and pushers
Measure case thickness with calipers. Replica cases run thicker because the movement needs a spacer ring. Check the crown engraving depth and the travel of the chronograph pushers. Replica pushers feel soft and stop short of the click.
Movement check
Current IWC chronographs in this family use caliber 69355, a column-wheel design with a 46-hour power reserve. Replica makers fit third-party automatic chronograph movements. Tells: the rotor rattles when you turn the watch, the chronograph seconds hand stutters, and the reset hand snaps back with a wobble. Ask the seller for a photo of the movement through the caseback and a video of the chronograph start, stop and reset.
Papers and price
A genuine watch ships with a warranty card that carries the reference, the serial and the dealer stamp. Check that all three match the watch. Price is the second signal. A replica listing sits below the market for the same reference in the same condition. Ask for the service history and the original purchase date in writing.