Pick a list of TLDs based on popularity and the number of domain name registrations. Pick both old and new TLDs, as well as ccTLDs (country code TLDs). Make up a word and register that word in all those TLDs. Choose one made-up keyword per domain name. Set up one website per domain name. Track the indexing of each website. Track the rankings of each keyword on each website. Track the progress and report the results.
http://www.circleid.com/posts/20200617-top-level-domain-bias-in-search-engine-indexing-and-rankings/
http://www.circleid.com/posts/20200617-top-level-domain-bias-in-search-engine-indexing-and-rankings/