Create an account, use it to excessively look for tea pots until you predominantly get ads for tea pots. Store the cookie[0] that encodes your user id. Delete the account.
After the advertised account retention period, try the cookie again in an otherwise squeaky clean session that gives random ads without the cookie:
Tea pots? Something survived.
No tea pots? No targetting.
(and yes, the onus to demonstrate your accusation well enough to convince a DPA is on you, IMHO as it should be)
[0] or whatever bits of data are necessary: try to recreate the tea-pot session after a logout without logging in officially to determine what's necessary.
They've improved their model none the less even if you delete your data after the fact.
An anecdote: I stayed with relatives for a few days one time, and my two-year-old niece really liked one particular music video on YouTube, so it got played on their TV box 3-4 times during my visit. Not on my laptop, but I used said laptop on their wifi (not logged into Google products, as I never am). When I got home, YouTube highly recommended that video to me after I watched an unrelated video, when there was no other reason to recommend it (I didn't know the artist, almost never watch music videos, and it wasn't anything all that popular).
My IP seems to be permanently associated with my YouTube habits. They’re even being sneaky about it, as in they’ll give you a default homepage and generic recommendations first, but watching any video similar to the previous viewing habits will bring back not just videos related to that one, but the entire history they’ve collected over the years (some of the topics I watch are completely separate and would never intersect normally, so the only way for them to both come up in suggestions is from previous viewing history).
I didn’t create an account, didn’t agree to any privacy policies, and am blocking any and all cookies just like they advise in their own privacy policy and yet I am still being tracked.
If they only use it for something where other factors affect the outcome (ad targeting, etc) they can still keep your data forever while hiding behind plausible deniability the other factors provide them (from outside, there’s no way to prove that a particular ad was served to you because of your data instead of another factor they use as input for the algorithm).