No single photo settles whether an IW371787 is genuine. The most reliable check is a hands-on inspection by a watchmaker with IWC parts access, backed by an IWC service center if the money is serious. The criteria that decide the outcome, in order, are: whether the reference exists in IWC's own records, how the case and dial are finished under magnification, what the movement and caseback actually show, and whether the provenance (papers, service history, price) holds together. Everything below is a filter that tells you when to walk away and when to pay for expert verification.

Verify the reference number before anything else

The IW371787 sits in the IWC Portugieser Chronograph numbering family. Confirm the exact reference, and what that reference is supposed to look like, against IWC's official catalog or a service center, not against a marketplace listing. Counterfeiters engrave real reference numbers on fake casebacks, so a number that exists is not proof of anything.

Strengths

• Free, fast, and needs no tools or disassembly.
• Catches invented references and re-engraved casebacks where the font, spacing, or depth does not match factory work.
• Exposes mismatches, such as the same reference advertised with dial colors or case materials IWC never offered together.

Limits

• A valid reference number tells you nothing about the watch in front of you.
• Secondhand listings and fan databases are incomplete and sometimes wrong, so absence of a match is a prompt to check further, not a verdict.

Use it as step one. Do it before you ask a seller a single question about price.

Case, lugs, and finishing

Look at lug edges, the transition between brushed and polished surfaces, and the inside of the lugs where a case is hardest to finish cleanly. Genuine cases hold a crisp line under a loupe; many counterfeits show rounded edges, uneven brushing, or polish bleeding onto surfaces that should be brushed.

Strengths

• Visible in hand and in high-resolution photos, with a loupe or a phone macro lens.
• Finishing shortcuts repeat across a production run, so one bad example usually means others exist.

Limits

• Wear, polishing, and aftermarket case work on a genuine watch can look as rough as a fake.
• Crown and pusher feel is subjective unless you have handled the real reference.

Dial printing, subdials, and hands

Under magnification, check the sharpness of printed text, the evenness of the subdial rings, the alignment of the hands at zero, and whether the applied numerals sit flat and evenly spaced. Fakes usually fail on the small stuff: fuzzy printing, misaligned markers, hands that reset slightly off.

Strengths

• Dial work is the hardest part of a watch to copy at low cost, so defects show up early.
• Photographs well, which makes it useful for a remote opinion before you travel to see the watch.

Limits

• Old or water-damaged genuine dials can print poorly too.
• Some dial variants exist in more than one generation, so know which one the reference should carry.

Movement and caseback

Ask for a clear photo of the movement through the caseback, and compare what you see with the calibre the reference should contain. Check engraving quality on the rotor and bridges, the presence of the correct jewels and screws, and whether the finishing looks applied rather than stamped.

Strengths

• Movement decoration is expensive to fake convincingly, so this is where many counterfeits fall apart.
• Service marks, dirt patterns, and mismatched parts can also reveal a Frankenwatch built from genuine and fake components.

Limits

• Photos hide a lot, and a swapped movement is easy to miss without opening the case.
• Opening a watch yourself voids warranties and risks damage. Leave it to a watchmaker.

Provenance, price, and paperwork

A price far below the going rate for the reference is the single most common signal. The same applies to a seller who has no box, no papers, no service history, and no explanation for where the watch came from. Serial numbers should match between the caseback and the documentation, and the paperwork should look like it was printed once, not scanned and reprinted.

Strengths

• Documentation and price are checkable before you meet anyone.
• A coherent story is a strong signal, and its absence is a strong warning.

Limits

• Counterfeit papers and boxes are widely available, so paperwork alone proves little.
• Genuine watches lose their boxes and cards all the time.

Professional verification, and who should use it

If you are spending real money on an IW371787, pay a watchmaker with IWC parts access to open the case and inspect it, or send it to a service center for an authentication and service estimate. That is the only step that turns a set of impressions into an answer.

Strengths

• Direct access to the movement, the case interior, and reference-specific specifications.
• Documented findings you can use in a return, a refund, or a dispute.

Limits

• Costs money and takes time, which is awkward during a fast negotiation.
• Service centers may refuse to service or return counterfeit watches.

Use professional verification for any purchase above your comfort threshold, and for any watch with an incomplete history that you intend to keep. Use the visual checks above when you are screening listings, deciding whether a trip is worth it, or figuring out whether a watch you already own needs a closer look.

Where this guide stops

Identifying a counterfeit is consumer protection. Sourcing one is a different matter: trafficking in counterfeit branded goods is illegal in the United States, and counterfeit watches commonly arrive with no valid warranty, unknown water resistance, and no parts support. If you want a Portugieser Chronograph look on a budget, buy a legitimate microbrand or an entry-level Swiss chronograph instead, and keep your money out of a market designed to deceive the next buyer.