How Could We Know if Animals Can Think Abstractly?

I have laid out in previous posts the evidence that animals are not capable of abstract thought. Furthermore, it is unclear what kind of abstract thought an animal could have, given that animals have no language in which to express abstract concepts. If an animal isn't thinking about a particular thing or about a word, what would the animal be thinking about?

But the question can be asked: How could we detect abstract thought in an animal?

Here's how. The experimental design necessary to de...