As you know, anyone can post information on a website. How do you know that the website you're looking at would be acceptable to your professor?
Take a look at five criteria on Evaluating Web Pages: Questions to Consider: Categories, from the Cornell University's website. Five Criteria give you five different ways to determine whether a site is acceptable.
Tip: if you're doing a search on Google, you can limit the top level domain by typing in site:edu or site:gov. These would be two ways of limiting your search to more credible sites.