It's important to consider the currency of the information you find on the Web. When was the information published or last updated?
Information posted in a blog or news format usually will have dates shown for each post or article. For static resources, you may see a "last updated" date (often at the bottom of the page.)
Currency is more important for some types of research than others. For rapidly-changing fields (e.g. science or popular culture), you will want the very latest information. If you are looking for the full text of Shakespeare's plays, however, using a recently updated source isn't necessary.