By default, hugo adds a generator meta tag in the home page of your website. This helps the Hugo team to build statistics about the popularity of the framework.
<meta name="generator" content="Hugo 0.xx" />
How can somebody use this tag to discover, with the help of search engines, websites that were built with hugo?
Not only for search engines…
For search engines, I just use "Powered by Hugo".
EXTRA: However, do not bother using Bing or any third-party search engine which uses Bing (like Yahoo, DuckDuckGo, Qwant, etc.) because Bing search currently have a major results issue wherein legitimate domain names are not appearing in searches EVEN though Bing Webmaster tools report the site was indexed and there are no issues at all. Microsoft is yet to acknowledge this major bug, it has been going on for more than a year now. The Bing search results currently is the worst and it affects all search engines relying on Bing.
Much better you use Google and any third-party search engines which relies on Google search. Or any third-party search engine with its own index database.
That is a good convension.
I was hoping that search engines would have an operator to find certain meta tags