Once you have written your SEO content, submit the URL to Google through Google Search Console.
Build backlinks, and wait for it to rank.
Your best proof of SEO-friendly content is its rank on Google.
You can also track your website content through Google Analytics, Google Search Console, and other SEO software, to check its progress over time.
Some key metrics to look at could be:
Impressions and clicks in Google Search Console
Keyword tracking on paid tools like Semrush and SE Ranking
Organic traffic on Google Analytics
Conversions and sales