Why Casio behaves differently to every other house
Casio is the one watch conversation in this catalogue that is mostly rational. The real product is inexpensive, so the question is not "can I afford the authentic one" but "does the listing behave like the thing I know". That makes it easier to evaluate and harder to be fooled - if you know what a G-Shock module does.
The module question is central. Button behaviour, display layout and backlight are functional facts a photo cannot prove. A short video answers it in ten seconds, which is why this is one of the rare categories where asking for motion is the efficient move.
Case and band finishing are visible: resin moulding quality, screw-back condition, and the way a bezel seat sits. Ask for macro frames on those three and you have covered most of what you can actually judge - and be honest that the water-resistance claim stays unverifiable.
Written for Casio specifically. The same discipline applied to a different house produces a different list - see the brand index.