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How to Spot a Fake G-SHOCK (and Where to Buy a Real One in the U.S.)
Short answer
Check three things: the module number on the case back should match the model (for example module 5611 on a GA-2100), every function in Casio's manual for that module should work, and a new watch should come from an authorized Casio retailer, which you can confirm in Casio's store locator. A price far below Casio's list price is a warning sign.
Check the case back and module number
Genuine G-SHOCKs print the model name and a four-digit module number on the case back: 5611 for the GA-2100 and GM-2100, 5689 for the GA-B2100, 5522 for the GA-700, and 3229 or 3525 for the DW-5600. A missing, blurry or mismatched number is a red flag.
Test the functions against Casio's manual
Download the manual for the module number from Casio's support site and try each mode: world time, stopwatch, timer, alarms and light. Counterfeits often skip functions or behave differently from the manual.
What about custom G-SHOCKs?
A custom G-SHOCK has been changed after it left Casio: a new colorway, a coated or metal case, or an aftermarket bezel. That is a different question from fake versus real. Ask the customizer which base watch is used and which parts are aftermarket; Bezelwright lists the base model and what changes on every listing, and is an independent customizer not affiliated with Casio.
FAQ
Where is the module number on a G-SHOCK?
On the case back, printed next to the model name. It is a four-digit number such as 5611.
Are G-SHOCKs on auction sites real?
Some are, some are not. Buy new watches from retailers listed in Casio's store locator, and check the module number and functions on any used watch.