Short answer

A replica Cartier Tank Must Large lives or dies on proportion. The genuine watch is a 33.7 mm by 25.5 mm steel rectangle, roughly 6.6 mm thick, with a silver opaline dial, polished steel sword hands and a fluted crown capped with a blue spinel cabochon. The movement is quartz. When a replica matches those dimensions within a fraction of a millimeter, prints the Roman numerals cleanly and sets something with real depth into the crown, it reads as the genuine article on a wrist from normal distance. When it is a millimeter thicker or the cabochon is cloudy resin, it does not.

One thing worth knowing before you shop: because the real Tank Must runs on quartz, the usual "the seconds hand ticks, so it is fake" argument does not apply here. A replica quartz movement beats at the same one tick per second. That removes the biggest tell people lean on with mechanical Cartier copies and pushes the whole evaluation onto the case, the dial and the finish.

Case proportions and thickness

Cartier lists the large Must at 33.7 mm long and 25.5 mm wide. Replica makers tend to get the width right and fudge the length or the thickness. On a flat rectangular case, an extra half millimeter of thickness shows from the side and changes how the watch sits, so ask for a caliper photo instead of trusting a product render. Look at the caseback as well. Cheaper copies bulge in the middle where the movement sits, and that bulge is the first thing you notice once the watch is on.

The lugs are short and the strap is narrow. Measure the lug width on the specific watch you are being shipped, or buy from a seller who states it, before you order a spare band.

Dial, hands and crystal

The dial is silver opaline, which is a soft matte silver. Not flat white, not a mirror. The Roman numerals should be black, sharp and evenly spaced, with clean serifs and no ink bleed. Check the railroad minute track: if the dots and lines are unevenly spaced or blur together at this size, the print is the weak point. "Cartier" sits in the upper half of the dial and "Swiss Made" runs along the bottom edge. Both should be small, crisp and centered. Crooked text is common on the lower tiers.

The hands are polished steel swords. A frequent flaw is a short hour hand that stops well before the numeral track. On the genuine watch the hands reach close to the printed track without touching it.

The original crystal is sapphire. A lot of replicas use mineral glass, which scratches sooner and reflects differently. Ask directly. If the seller cannot answer, assume mineral.

Crown, cabochon and caseback

The crown is fluted and set with a blue synthetic spinel cabochon. It is a tiny part and a large tell. A real stone has depth and shifts shade as light moves across it. Resin or glass looks flat, milky or oversaturated. The cabochon should also sit upright and flush in the crown, not tilted or standing proud of the metal.

Caseback engraving is the other close-up test. On a good replica the lettering is cut deep and even, with the reference number, steel marking and water resistance stamp all present. Laser etching that looks grey and shallow is a downgrade.

Movement and battery

You are buying a quartz watch, and so is everyone who buys the original. Most replicas run a Japanese caliber such as a Miyota or a Swiss Ronda. From the outside they are indistinguishable, and both are easy for a local watchmaker to service or to fit a new battery in. The practical difference is battery life and long-term availability of the movement. Ask which caliber is inside, then judge whether the price lines up.

Strap and buckle

The strap is where most replicas save money. Genuine is grained calfskin with a steel ardillon buckle. Replica straps are often stiff, shiny or thin, and the buckle can look cast rather than machined. This is also the easiest thing to fix. A decent aftermarket strap in a similar grained finish lifts the watch more than almost any other upgrade, and it takes five minutes.

What I check before buying

• Caliper photos of length, width and thickness, not marketing images.
• A macro shot of the dial, to judge numeral print and the minute track.
• A close-up of the crown cabochon shot in daylight.
• The caseback engraving, lit from the side so the depth shows.
• Movement brand and caliber.
• Strap material and buckle photos.
• A written return window.

Bottom line

The Tank Must Large is one of the friendlier Cartier models to buy as a replica, because the design is simple and the movement is quartz on both sides. Spend your attention on the case thickness, the dial print and the cabochon, in that order. Get those three right and the rest is a strap away.