How do you check if a website is registered?

I'm curious, could you walk me through the process of checking if a website is registered? Is there a specific tool or database I should use? How can I verify the authenticity and legitimacy of a site's registration? Are there any common red flags to look out for when conducting this check? Additionally, what information do I typically get from a website's registration details, and how can I use this information to protect myself online?