Most sites have "About Pages" - in the Menu, they will be called "About Us" or "About [Something]" or just About. If they do not, they may have a page for "Our Goals", or "Our Mission", or "Mission and Vision" or even "Mission, Vision and Values". It’s also useful to see if the website has a “Legal” or “Disclaimer” section. Many satirical websites disclose this information in those sections.
Sites that provide no information about why they exist are suspect. If you cannot find any About Us information, it is probably time to consider the site unreliable.