A few ways I can think of to check:
1) Referrals in Google Analytics
2) 3rd party tools (such as Majestic) that disclose inbound links
3) Links to Your Site report in Search Console
4) the "link:" command in the search box
5) Crawl the suspected domain (if you know it) with a tool (such as Screaming Frog) and see what outbound links the domain has.
New advertising option:
A review of your product or service posted by a Sterling Sky employee. This will also be shared on the Sterling Sky & LSF Twitter accounts, our Facebook group, LinkedIn, and both newsletters.