Websites embed identifiers from third-party services: analytics, advertising, fonts… The same identifier on two websites often gives away a common owner. Search for an identifier (for example LS-12345-6) or a domain name.

In real life: DNSlytics, SpyOnWeb, BuiltWith (Google Analytics, AdSense, Google Tag Manager identifiers). Ignore identifiers shared by thousands of websites: only rare identifiers link two sites.