The most telling check on a fake IW371758 is the chronograph itself. Start it, let it run a few seconds, then reset: on a well-built watch the 30-minute counter at 12 and the running seconds at 6 both snap back to zero in one motion, and each pusher has a short, firm travel. On a poor replica the reset drags, one counter lags behind, or a pusher goes soft halfway through its stroke. That single test, plus the five checks below covering dial print, case finishing, crystal, caseback, and hardware, is the criteria set that separates a wearable first copy from a return. Reference-level specs vary between Portugieser Chronograph generations, so confirm case size, calibre, and dial details against IWC's own data for IW371758 before you judge any watch.

1. The chronograph reset test

Run the chrono for thirty seconds, stop it, then reset. Repeat three times. You are looking for a synchronized, instant return of both counters and a pusher feel that stays consistent.

What it catches

• Cheap quartz or low-grade automatic chronograph modules that reset with a visible lag
• Repurposed subdials, where a 24-hour hand stands in for the 30-minute counter
• Loose pusher springs and stems that bind after the first press

Where it falls short

• A good replica built on a Valjoux-style movement can pass this test with no trouble
• It says nothing about dial print, case finishing, or crystal quality
• You need the watch running, so it fails on a dead battery or an unwound mainspring

Use it when you can handle the watch or when a seller will send a short video of a full start, stop, and reset cycle.

2. Dial print, indices, and hands

The Portugieser dial is a print-heavy design: thin applied numerals, a fine railway minute track, and long leaf hands. On a replica the giveaways sit at the edges. Look at the subdial rings, which should be recessed into the dial rather than printed on the surface. Check the letter spacing on the brand name, the thickness of the numeral strokes, and whether the hands reach the minute track without overhanging it.

Strengths of this check

• Needs no tools and no movement access
• Catches the most common replica shortcut, which is printing subdial rings instead of cutting them
• Works from clear photos, so you can screen a seller before paying

Limits

• Photos hide gloss level and print relief
• Legitimate production changes exist across the line, so a difference is not proof of a fake

Use it when you are buying from photos and need a fast way to reject weak listings.

3. Case shape, finishing, and dimensions

Measure the case with calipers if you can. Portugieser Chronograph cases sit close to 41 mm across recent generations, with small variation between earlier and later versions, so a watch that measures 43 mm is not the reference it claims to be. Run a fingernail along the caseband and lug edges: genuine finishing is polished and brushed in defined zones with a crisp transition line, while replicas blur the border or polish everything.

Strengths of this check

• Hard numbers are hard to fake
• Catches oversized cases built on a generic shell

Limits

• Case dimensions alone do not confirm authenticity on a well-copied shell
• Requires calipers or a trusted measurement from the seller

Use it when a listing quotes no measurements, which is often the case with first-copy sellers.

4. Crystal and dial legibility

Genuine sapphire on this line carries anti-reflective treatment that keeps the dial readable from steep angles. Replicas often use single-sided coating or none, which shows as a milky sheen at 45 degrees and a faint purple or green tint at the crystal edge. Tilt the watch under a lamp and watch how the reflection behaves as you move.

Strengths of this check

• Quick, and it needs only a light source
• Reveals cheap crystal substitutions that photos tend to hide

Limits

• Coating quality varies on genuine watches with age and wear
• Some good replicas now use double-sided AR sapphire

Use it when you are judging a watch in person, since it takes about five seconds.

5. Caseback engraving and movement view

Look at the depth and consistency of the engraving. Genuine laser marking is shallow but uniform, with clean edges on the reference and serial text. Replica engraving tends to be deeper, uneven, or slightly blurred at the letter ends. On an exhibition back, check that finishing under the rotor is applied rather than stamped, and that the rotor text is etched with sharp borders. Earlier Portugieser Chronographs used a Valjoux-derived calibre, while later ones run IWC's in-house chronograph movement, so confirm which generation IW371758 belongs to before you judge what you see.

Strengths of this check

• Movement finishing is the costliest thing to copy well
• Serial and reference text give you something to compare line by line

Limits

• Solid casebacks remove this check entirely
• Decoration plates over a basic movement can look convincing in photos

Use it when the watch has a display back and the seller can supply a close, well-lit movement photo.

6. Strap, buckle, and hardware

Check stitching density and evenness, the taper of the strap toward the buckle, and the crispness of the signed buckle or deployant. Replica straps often use embossed leather rather than alligator, with wider stitch spacing and a buckle that feels light in the hand. Look at the spring bars too: mismatched or thin bars are a sign of a budget build.

Strengths of this check

• Catches cost cutting that the dial hides
• Strap and buckle are often the first parts a previous owner replaced, so condition tells you about ownership

Limits

• Straps are consumables, so a replacement is normal on a genuine watch
• Aftermarket straps on a real watch are common and not evidence of anything

Use it as a tiebreaker when two watches pass every other check at a similar price.

Which check to lead with

If you can only do one thing, run the chronograph reset test and ask for it on video. If you are buying from photos alone, lead with dial print and subdial construction, then ask for caliper measurements. If you are buying a first copy rather than judging a genuine watch, weight the seller's return window and the quality of their actual photos above paperwork, since box and cards mean little on a replica and a clear return policy means a lot.