"Through what technique did they discern the difference between mere correlation, and a correlation with exploitable causality?"

Evolution - that is, assumptions that are accurate are favoured since they are more likely to lead to the animal surviving, vs embracing spurious correlations which are likely to get you killed.

Of course, this can break down if we try to apply the cognitive rules of thumb we've evolved to new domains outside our original evolutionary scope - our difficulties in thinking about statistics and probability are a great example of this. The math is trivial, but it just doesn't fit our brains very well without a great deal of cultural scaffolding.