Short answer

A fake Cartier Tank Must Large gives itself away in four places: the crown cabochon, the dial print, the caseback engraving, and the movement. The genuine Large is a 33.7 mm by 25.5 mm quartz watch, about 6.6 mm thick, with no seconds hand. Get those numbers and that one omission into your head and most counterfeits fall apart in under a minute.

Check the case first

The Tank shape is unforgiving. The vertical brancards on a real watch are flat planes with a crisp polish line running the full height, and the corners where they meet the case are sharp rather than rounded off. Fakes fail here first. The brancards bulge slightly, the corners look soft, and the whole case reads thicker than 6.6 mm once calipers touch it. If a seller only shows angled photos, ask for a straight-on shot with a ruler in frame.

The crown and the cabochon

Cartier sets a synthetic blue sapphire cabochon into the crown. On the real thing it is a dome, deep and slightly translucent, seated in a polished crown with a defined edge. Counterfeit crowns tend to carry a flat or cloudy stone, sometimes glass or resin, often sitting proud of the surface. Under a loupe you can see the difference in how light settles inside the stone.

The dial tells

Two things matter. First, no seconds hand. The Tank Must Large runs on quartz with two hands, period. A "Large" with a sweeping or ticking seconds hand is the wrong watch. Second, the print. Look at the Roman numerals and the chemin de fer minute track: on a genuine dial the lines are fine and even, the "Cartier" at twelve sits in the right weight and spacing, and the "Swiss Made" at six is small and sharp. Cartier also hides a secret signature inside a numeral on many of its dials, so a loupe and a steady hand beat any photo comparison you find online.

Caseback, reference, and paperwork

Engravings on the back should be cut into the metal, not layered on top of it. You want the reference number, the serial, "Swiss Made", and the water resistance rating, all struck at the same depth. The serial on the case should match the paperwork. Photocopied certificates, a missing serial, or a seller who blocks the number in photos are all reasons to walk away.

What "1:1" means in practice

Any listing promising a perfect copy is selling you a watch that has never sat under a loupe next to the original. Replica makers copy the outline well. They do not copy the dial printing tolerances, the crown geometry, the finishing on the bracelet links, or the way the caseback engraving catches light. Buy secondhand and you are paying for a watch that a Cartier service center will refuse to touch and that a reseller will not buy back.

How to verify before money changes hands

• Ask for the reference and serial in writing, then confirm both with Cartier.
• Request a straight-on, well-lit photo of the dial and a close shot of the crown.
• Ask for caliper measurements: 33.7 mm across, 25.5 mm tall, 6.6 mm thick.
• Meet at a Cartier boutique or an independent watchmaker and have the caseback opened.
• Treat steep discounts on a "new" Tank Must as a signal rather than a bargain. Steel Large models sit in the low-to-mid $3,000s at retail depending on strap or bracelet.

None of this needs a loupe and ten years behind a bench. It needs you to slow down and check the few things replica makers still cannot get right.