As a very rough start you could use some website grading tools to see what issues you may have and then check your site on tools like ahrefs or semrush
Your website rankings will drop fast, this would be one sign for wrong SEO. Also, you can check your website SEO with various SEO checker tools online.
It's hard to analyze off-page seo, you'll need tools like Semrush or Ahrefs and they are not free. For on-page SEO You can use Pulno, analyzes and explains potential errors on the website.
You should check your website by using SEO tools whether any keyword of your website has ranked or not. And if you've built any poor quality backlinks or not. If your website has poor quality backlinks and any keyword has not ranked in Search Engine Result Pages (SERPs), then SEO on your website is not good.