The counterfeit you are most likely to run into is a fake Blancpain Fifty Fathoms Automatique 5015, with the Bathyscaphe close behind. Five checks separate a genuine diver from a copy: dial printing and logo geometry, the sapphire bezel and lume, movement and caseback engraving, case and crown finishing, and the paperwork paired with the asking price. All five can be done in hand, without a case opener.

Dial printing and logo geometry

This is the first thing to read under a loupe or a phone macro lens. Blancpain uses a fine serif wordmark with consistent stroke weight, and the text sits on a clean baseline.

Pros

• A well made copy can reproduce the overall layout, so the dial looks correct from arm's length.
• The Fifty Fathoms name and depth rating are usually printed in the right zones.

Cons

• Letterforms on fakes drift: thick serifs, uneven spacing, or a wordmark that sits too close to the 12 o'clock marker.
• Date apertures are cut wrong, often too wide or with a sharp square edge instead of the correct proportion.
• "Swiss Made" at 6 o'clock is crowded or sized differently from the reference.

Best for: a first pass at a distance, or a listing photo you cannot inspect in person.

Sapphire bezel and lume behavior

Blancpain fits a domed sapphire insert on the Fifty Fathoms, and the numerals and markers are filled, not painted on top.

Pros

• Some higher end replicas do use a real sapphire insert, so the material alone is not proof.
• Marker positions on good copies follow the genuine layout fairly closely.

Cons

• Cheaper copies use mineral glass or coated acrylic, which shows scratches and a flatter reflection.
• Lume charged under a lamp fades in minutes on a fake and holds far longer on the real article.
• Bezel action has back play or a gritty click instead of a firm, even step.

Best for: buyers who handle the watch in daylight, where insert material and reflection are obvious.

Movement and caseback engraving

The 5015 has a display back, which makes this the most decisive check.

Pros

• A genuine rotor carries clean engraving and the finish is consistent across the bridges.
• Serial and reference numbers sit in the expected places.

Cons

• Fake rotors show shallow laser engraving and a rough, unpolished underside.
• Jewel count, shock setting, and bridge shape often belong to a generic automatic movement.
• Caseback text fonts are heavier than the genuine stamping, and the caseback sits proud of the case.

Best for: any deal where you can see the movement before money changes hands.

Case and crown finishing

Blancpain alternates brushed and polished planes with crisp transitions, and the crown carries a deep logo relief.

Pros

• Good replicas copy the overall case silhouette and lug width.
• Bracelet or strap hardware can look close in photos.

Cons

• Bevels on fakes are rounded and soft, because the edges are polished by hand after machining.
• The crown logo is shallow and loses definition under a loupe.
• Lug holes, crown guard shape, and caseback thickness are usually off by a fraction that shows on a caliper.

Best for: in person inspection, especially with a caliper and a strong light.

Paperwork and asking price

Documentation and price are the fastest filters, and they cost you nothing to check.

Pros

• A genuine set has a matching serial on the card, the caseback, and the movement.
• Sellers of real watches answer serial questions without hesitation.

Cons

• A luxury diver at a fraction of market value is the clearest signal of a fake, or of a stolen watch.
• Blank warranty cards, reprinted boxes, and sellers who refuse a serial photo are common with counterfeits.
• Copy text on cards and manuals has spelling and layout errors.

Best for: an opening question before you travel to see a watch, since a refusal to share the serial ends the conversation.

Verify through the brand or an independent watchmaker

When two or more checks raise doubt, stop. A Blancpain boutique or an authorized service center can confirm a serial and service history, and an independent watchmaker can time the movement and confirm the caliber family. Budget for that step before you pay, because a counterfeit diver has no parts support, no resale value, and no water resistance you should trust near a pool.