That overly broad question is based on the false premise that Web sources are more or less of equal quality.
Instead, ask two targeted questions:
1) How much trust — or how little trust — should one place in specific Internet sources?
2) what are meaningful criteria for answering that kind of question?
But you can learn to evaluate the credibility of Internet sources systematically.
You can do this evaluation by answering some basic questions.
* Who is the author? (individual, corporate, pseudonymous…)
* How qualified is the person or organization responsible for the website or e-mail communication?
* Are primary and secondary sources for claimed facts cited clearly enough?
* Is this website logically organized?
Read other Vincent Pollard's thoughts about the credibility of Internet information in his article.