There are a multiple ways to tell if a website has structured data. One way is to look at the source code. If you see tags like Microdata, RDFa, or JSON-LD, then the website likely has structured data.
Another way to tell is to use the Google Structured Data Testing Tool. You can enter a website URL, and the tool will analyze the page to see if it has any structured data.If a website has structured data, it may have rich snippets in the search results. Rich snippets are special search results that include additional information, such as the author, rating, or price.
To see if a website has rich snippets, do a Google search for the website. If you see rich snippets in the search results, then the website has structured data.