There is no universal standard of reliability when getting answers to questions online. It depends on the expertise of the user base that frequents the website in question. Websites such as Yahoo! Answers, WikiAnswers and other public forums have few requirements for becoming a member, so there is little screening.
People who need answers to specialized questions regarding medical, financial or legal issues should consult websites operated by professionals in the field. There are ways to determine a website's legitimacy. For example, information that is not the product of the webmaster should include clear attribution, with a link provided if possible. Also, professional websites should have the user's credentials easily visible and verifiable.