The fastest answer: a Vacheron Constantin case is genuine gold only when it carries the Swiss precious-metal hallmark for 18k together with the 750 stamp, and the metal passes a density or XRF check. Plated copies fail one of those two tests in nearly every case. The checks below are ranked by what they actually prove. Hallmarks and stamps come first because they are a legal requirement, not a design choice. Weight and density come second because plating cannot fake mass. Wear and color, movement finishing, and bench testing round out the list. The first three checks work from clear photos. The last two need the watch in hand.

Hallmarks and Caseback Stamps

Swiss 18k gold cases carry the St. Bernard dog head, the official Swiss mark for 750 gold, plus a 750 or Au750 stamp and a maker's mark. A genuine Vacheron Constantin case also shows a reference number and a case serial, struck cleanly and placed in line with the caseback engraving. Copies reproduce the brand name and skip the legal marks, or stamp them in places the real case never has them.

  • Pros: no tools needed, readable in a sharp photo, easy to compare against archive images and auction listings.
  • Cons: hallmarks are shallow, so polishing and years of wear soften them, and a careful counterfeiter can cut a mark that passes at arm's length.

Use this check first when you are screening a listing online or looking at a watch across a counter, because it costs nothing and eliminates most obvious copies in seconds.

Weight and Density

An 18k gold alloy runs near 15.5 g per cubic centimeter, so a solid gold case feels heavy and bottom-weighted for its size. A plated brass or steel case with the same dimensions lands well under that. Weigh the watch head on its own and compare it against a known figure for the same reference and case size, or measure water displacement to get density directly. Plating and polishing cannot add mass.

  • Pros: objective, repeatable, and hard to fake without using real gold in the first place.
  • Cons: needs a scale accurate to a tenth of a gram, and bracelet or strap removal changes the comparison, so match the setup you measure against.

Use this when the caseback looks correct but the watch still feels light in the hand, or when a seller refuses returns and you want a physical number to argue with.

Wear, Color, and Plating Failures

Plating wears through where a case rubs: lug edges, crown guards, the caseback rim, and the underside of the bezel. Look for a second metal color showing in those spots, and for a surface that shifts from yellow toward orange or pink. Solid 18k gold does not flake, bubble, or reveal a different core, and a scratch on real gold stays gold inside the scratch.

  • Pros: visible in close-up photos, needs no equipment, and a plated case shows these marks within months of regular wear.
  • Cons: a heavily polished genuine case can also lose definition, so read the wear pattern together with the hallmark and the weight.

Use this when a seller sends close-ups of the dial but avoids the lugs and caseback edges, which is where plating gives itself away.

Movement, Dial, and Papers

Open the caseback if you can. Vacheron Constantin finishes its movements to a high standard: Geneva stripes, polished bevels, circular graining, and the Maltese cross on the rotor or bridge where the caliber calls for it. A gold case wrapped around a rough, unmarked, or quartz movement is a warning sign. Papers and service records should match the case serial and reference, not just the model name on the dial.

  • Pros: finishing is expensive to copy, and serial matching links the case, movement, and paperwork into one story.
  • Cons: opening a case risks scratches and moisture seals, and paperwork alone never proves metal content.

Use this when the asking price sits far below what a gold reference normally trades for, since that gap is the single strongest signal that the case is not solid gold.

Bench Tests: XRF, Acid, and Specific Gravity

An XRF gun reads alloy composition in seconds and is the most reliable non-destructive test available at a jeweler or pawn shop. Acid testing works but marks the metal and needs a hidden spot on the caseback rim. A specific gravity check needs no chemicals at all: weigh the head dry, weigh it suspended in water, then divide. A magnet test only rules things out, because gold is not magnetic. A magnet that sticks proves the case is not solid gold. A magnet that does not stick proves nothing.

  • Pros: XRF gives a printed alloy percentage you can keep, and specific gravity needs only a scale and a glass of water.
  • Cons: acid leaves a permanent mark, XRF machines are not everywhere, and density readings drift if the case has a steel movement ring or a mixed-metal bracelet.

Use XRF when a jeweler offers it before purchase, and fall back to specific gravity when no equipment is available and the case is bare.

Buying Copies Without Crossing the Line

Replica watches sold openly as replicas are a separate market from fraud. Trouble starts when a copy is listed as genuine, or a gold-plated watch is described as solid 18k, which is precisely what the phrase fake Vacheron Constantin gold describes. Passing a copy off as real invites fraud claims and, in the United States, federal counterfeit rules. If you buy a replica to wear, keep the listing text, the receipt, and any description that states replica, so the metal claim never becomes yours to defend.