Comment by rgovostes

Here's what I would accept:

Go to a public library without any electronic devices and pick up a print copy of a magazine with some ranked list of products or brands. Select a handful that you don't think would be targeted towards you. For example, beauty products, childcare products, cruises marketed towards seniors, etc.

Over the course of a few weeks, use Siri to send text messages mentioning these brands, add them to your Reminders list, etc. Just don't allow it to redirect you to any apps or web searches, since that would invalidate the experiment.

Browse Apple News daily, which uses Apple's internal ad service. If you see any ads related to at least 2 of your canary brands, you have plausible evidence that your Siri interactions have been used for ad targeting.