The US legal system has this interesting "discovery phase" where the parties basically get to request all possibly relevant data from the other side. They'll try to find incriminating mails or other documents.

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.