While writing OP, I was curious enough about how they could solve Clever Hans that I went and read the original Pfungst book. I recommend it: his thoroughness & cleverness in studying Clever Hans (as well as the history of previous 'talking animals') ought to be better known than Clever Hans himself, IMO. (Brief review: https://gwern.net/review/book#clever-hans-pfungst-2011 )
