The short answer
A fake Cartier Tank Must 31mm usually fails in four spots: the dial print, the crown cabochon, the caseback engraving, and the crystal. Because the real watch runs on quartz, a faker does not have to copy a mechanical movement, so the cosmetics carry all the weight. Check those four things first and most bad replicas sort themselves out in under a minute.
What the real one is
The Tank Must is Cartier's steel entry point into the Tank line. The 31mm case class has a thin profile, a fluted crown set with a blue cabochon, sword hands, and a dial built around roman numerals and a railroad minute track. It is a quartz watch. Cartier lists quartz on the spec sheet, and that is not a downside here: it keeps the case slim and the watch easy to live with. The strap is usually grained leather, and there is a steel bracelet version.
The tells that matter most
Dial printing
- Numerals on the genuine dial are thin and evenly spaced around the track. Replicas fatten the strokes and crowd the spacing near the bottom.
- The secret signature: Cartier hides its name inside one of the numerals. On fakes it is missing, printed far too heavy, or placed in the wrong numeral.
- The minute track should be crisp. Blurry or doubled print is a hard fail.
- The dial surface is silvered or lacquered with a soft sheen. Cheap reps look chalky under light.
Crown and cabochon
- The cabochon is a smooth dome with depth, no bubbles, no cloudiness. Plastic beads read flat, milky, or oversized.
- Fluting on the crown is fine and even. Coarse grooves give it away.
- Hands are blued steel. Under light they read deep blue-black, not purple or painted metallic blue.
Case and back
- Engraving on the back is crisp with sharp edges. Laser engraving on fakes sits shallow and soft, and the font drifts from the real one.
- Polished and brushed surfaces meet in clean lines. Look at the lug tops, where finishing shortcuts show first.
Movement and crystal
Quartz is quartz, but the seconds hand still tells you something. On a genuine watch it lands on the printed markers. On cheap fakes it stutters or sits between them. The crystal on the real watch is sapphire, so it stays clear edge to edge with almost no distortion. Mineral glass on a fake shows a green tint at angles and softens the dial near the edges.
Where replicas land
Replica quality runs in bands. Under about $150 you get mineral glass, a printed cabochon, and dial text that misses the font. The middle band adds sapphire and a better dial, but the cabochon and caseback still slip. The top band fixes the crown, the dial print, and the engraving, and those watches hold up to a casual look across a table. None of them survive a loupe beside a genuine one.
Before you buy anything called genuine
Price is the fastest filter. If a steel Tank Must 31mm is offered at a fraction of current retail, assume fake until documents and a service history say otherwise. Ask for the reference number and check it against the model, ask for movement photos if the back opens, and prefer sellers with a return window. On the replica side, ask which crystal, which cabochon material, and whether the dial is pad printed or stamped. A seller who cannot answer those three questions is moving whatever is cheapest that month.