Quick visual checks
Crystal etching: post-2002 Rolex has a tiny laser-etched coronet at 6 o'clock visible only under 10x loupe and good light. Missing or sloppy = fake. Case-back: should be smooth and unengraved on most modern references; references with exhibition case-backs are extremely rare and limited. Engraved case-backs (logos, slogans) on a sports Rolex are an immediate fake flag. Cyclops: the date magnifier should be exactly 2.5x; if the date doesn't fill the cyclops aperture, fake.
Movement-level checks
If the case-back can be opened (don't do this on a piece you might return), the movement should be signed Rolex at multiple points: rotor, balance bridge, automatic device. The finishing should include circular Geneva striping and polished sinks on visible bridges. The serial number on the case should match the model and year (Rolex publishes serial-by-year tables online); many fakes have implausible serial numbers.
Why DIY isn't enough anymore
Modern super-clones (Clean Factory, VS Factory, ZF Factory) replicate everything: laser-etched coronet, correct cyclops magnification, case finishing that passes 10x loupe inspection, and movement architectures that LOOK like Rolex calibres. The differentiator is movement timing: super-clone movements drift wider than COSC spec within months. A timegrapher reading is the cheapest reliable test; any AD or watchmaker can do this in minutes for free or CHF 20.
When to use a professional service
For purchases over CHF 5,000, ALWAYS use a professional authentication service. Pre-purchase: Watchfinder, WatchBox, Bob's, Hodinkee Shop all sell with their own authentication and 12-24 month dealer warranty. Post-purchase: independent watchmakers can authenticate for CHF 100-300. Brand service centres (Rolex Service Centre) will inspect and authenticate but not give written reports. The cost is trivial relative to the purchase risk.