Some things to consider:
Authority: Who is the author? Does the person have credentials in this area or a record of research?
Source: Does the page come from a reputable institution, agency, or organization?
Currency: Can you tell when it was published? Is this a topic that's likely to change often?
Research: Does the web page cite other studies?
Objectivity: Is this just an opinion piece, or does it rely on documentable evidence?