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> holding the right button for over 3 seconds in the main timekeeping mode; this will lead the display to show "CASIo", as a test for authenticity

https://en.wikipedia.org/wiki/Casio_F-91W#Counterfeits

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How difficult would be for a fraudster to implement such functionality?

The question should be, how much would it cost to make a convincing fake using custom circuitry?

The genuine product is so cheap, you probably can't compete with it unless you use a mass-produced generic movement.

You can't replicate its LCD or UI with some "generic" digital watch movement. Fakes have a reimplementation of the Casio module and many do output CASIo. The fakes are mass produced. There are a few identifiable families of fakes with particular errors in their production.

Mine has an additional tell in the printing on the face: a tiny, precise U under the word RESIST. On fakes, which are usually poorly printed, the U is much bigger.

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