This raises a very difficult question: Given the results are personnalized and the algorithm a secret, how can we bring proof? Same question for many companies who can hide behind the absence of measurable data, because they keep it secret.
There is a decent likelihood the EU court will mandate Google to provide the court this information. With any luck, some of it will end up public, though Google will probably try to leave the EU as a whole before they'd give up their search formula.
