[Someone]

It doesn't have to know what it looks like; it only needs to "know" what its predator's food looks like, or rather, what its predators are more likely to hunt.

And that probably is "know", as in "built into its genes through evolutionary trial and error". It is not likely this is something mothers learn their children, or that they adapt the looks of their decoys within days if researchers let new predators loose (either might be true, but would truly be surprising)