It was somehow both immediately obvious and also subtle. My girlfriend eventually confirmed that something didn't seem right with it, so we started buying back-to-back orders alternating from grocery stores vs. amazon. Every once in awhile we'd get non-counterfeit cereal from Amazon but mostly it was "fake". Or at least diverted from another global region where the recipe/ingredients are somewhat different.
I always wanted to do some quantitative analysis like density or fill size or precise box dimensions or spectrometry of the ink on the boxes. But at the time I was just trying to convince myself that I wasn't crazy, and eventually I was 100% sure so I stopped experimenting before bothering to collect quantitative data.