Short answer
A replica Cartier Santos two tone lives or dies on the small stuff: the two-tone line on the case and bracelet, the screw heads on the bezel, the dial print, and the bracelet fit. If those four look right in normal light, the watch reads well on the wrist. If any one is off, no movement upgrade saves it.
Case shape and the two-tone split
The Santos has a square case with a soft curve at the lugs, and the gold section should follow that curve without a wavy edge. I check the transition at the bezel first. On a weak replica the gold strip looks painted on and the border between steel and gold is fuzzy. On a better one the line is crisp and the gold sits flush. Look at the case from the side too. The gold should wrap the bezel and mid-case in a way that matches the bracelet end link, not stop short.
Gold tone and plating
Two-tone is harder than full steel because the gold has to match across three areas: bezel, case flanks, and bracelet links. A yellow tone that looks brassy under warm light is a red flag. So is a rose tone that shifts pink under fluorescent light. Plated gold also wears at the clasp and the caseback edge, so check those spots for dull gray show-through. Solid gold parts are rare in replicas at this price, and a seller who claims solid gold on a two-tone Santos is worth a hard look.
Bracelet, screws, and fit
The Santos bracelet has visible screws and a quick-release system on the modern model. On a replica, check that the screw slots line up, that the heads sit flush, and that the links move without grit. A bracelet that rattles or has sharp edges at the end links will feel wrong right away. I also check the clasp engraving and the push-button action. Loose buttons and shallow stamps are common tells.
Dial, crystal, and hands
The dial print should be sharp under a loupe, with even spacing and no ink bleed. The Roman numerals should look printed, not stamped. The crystal should sit close to the bezel and show little distortion at an angle. Hands should be clean at the edges, with the blue steel tone consistent. A date wheel that sits off-center or shows a bold font is an easy catch.
Movement and setting
Most replicas in this range run a clone automatic or a quartz. Wind it and set the time. The crown should feel smooth, not sandy, and the date should flip with a clean click. A movement that hacks and hand-winds is easier to live with. Ask about service history if it is a used piece, and expect a power reserve in the 38 to 42 hour range for a decent automatic.
What I would skip
Skip any listing that hides the clasp, shoots only in warm light, or calls the watch "1:1" without close-ups. Skip sellers who will not show the caseback and bracelet screws. And remember that a replica is still a trademark issue in many countries, so know the rules where you live.