An external study to evaluate whether Google is using fingerprinting would be some work, but pretty doable. Targeted advertising is generally very blunt: if someone thinks you're especially interested in a valuable category they'll often pay a lot to advertise to you. So you could set something up where test browsers visit pages related to high-value categories (mattresses, asbestos cancer, credit cards, ...), clear client-side data, and then visit a site that loads ad scripts only from Google (to make sure you're not getting someone else's fingerprinting) and see whether the ads differ from a control group that never visited those pages.