They could also work with statistics. Google being google, they most certainly have enormous amounts of data on their recruitment process. I'd even bet Google has algorithms to uncover discrimination in its hiring – biases are bad for business, after all. They may not have had age discrimination on their radar, but whatever they have on race/gender/dog-vs-cat-owners should be easily adaptable.

My closest guess is that this lawfirm is going to provide an X amount of cases that passed screening etc and have relevant experience to the job as described in its advertisement.

Then its really up to the Judge and the jury if such a case has a jury to decide.