I generally try and buy counterfeit stuff, that's my thing.

But I have never checked my Casio F91W is counterfeit, so I figured it's a good time, I used this -

https://www.youtube.com/watch?v=pGuJAJ0I1Cg

My watch says made in Japan, so I guess it's a genuine fake. But it is full of water, so that bit sucks.