The short answer is the movement. A. Lange & Söhne builds the Saxonia line in gold and platinum, with hand-wound or in-house automatic calibers finished to a level the replica trade does not touch. If a watch marked Saxonia has a steel case, a Miyota or Seagull engine behind the back, or a slab profile that reads like a diver, it is not a Saxonia. It is a cheap watch wearing the name.

What the Saxonia line actually is

Saxonia is Lange's quiet family. The current range runs from the Saxonia Thin, which is a two-hand dress watch, through the classic Saxonia with small seconds at 6, to the Saxonia Automatic, Saxonia Moon Phase and Saxonia Annual Calendar. Cases land between 35mm and 40mm depending on reference, and the Thin sits under 6mm thick. Case metals are pink gold, white gold, yellow gold and platinum. Reference numbers follow a pattern like 219.032 or 380.032, and Lange publishes every one of them with its caliber on the brand's own site.

The five checks that matter

• Case metal. There is no steel Saxonia in the regular collection. If the case is steel, or the seller calls it "gold-plated," stop there.
• Profile. A Thin measures roughly 6mm. Fakes built on generic movements run 9mm to 11mm because the movement dictates the case. Measure, or ask for a caliper photo.
• Dial printing. Look at the lettering under the 12, the spacing of the brand signature, and where the country-of-origin text sits. Compare against a macro shot from Lange's own catalog. Fake dials run thick, soft or slightly crooked.
• Applied markers and hands. Gold markers, sharp edges, clean polishing. Heat-blued hands on references that should have them, not painted blue.
• Papers and price. A genuine Saxonia in gold starts in the low five figures used. Anything priced in the hundreds is a replica, whatever the listing says.

Reading the movement through the caseback

The Saxonia Automatic uses caliber L086.1, so a rotor alone proves nothing. What proves something is the finish. Lange movements show an untreated German silver three-quarter plate, screwed gold chatons, blued screws and a balance cock engraved by hand with a floral pattern. Every engraving is slightly different because a person cut it. Replica movements show stamped or laser-etched decoration, plated base metal, and a balance cock that looks printed rather than cut. The hand-wound Saxonia Thin uses caliber L093.1, and it is one of the thinnest production movements around. No replica house has copied it, because copying it is the hard part.

Serial numbers, papers and the honest part

Ask for a movement photo, a caseback macro, the reference number and any service paperwork. A seller who will not send a movement shot is telling you something. Genuine examples come with a box, a booklet and a numbered guarantee card, and the serial should be consistent across the caseback and the paperwork.

Here is the part people skip. A high-end replica or homage is a different product from a watch sold as genuine. The problem starts when a replica is described as real, priced as real, or resold to someone who does not know. Buy a replica because you want the look at that price, and know exactly what you are holding. Buy a Saxonia because you want the movement, the finishing and the ownership record. Do not mix the two up, and do not let anyone mix them up for you.