Fortunately currency of information is one of the easiest parts of evaluating a source. Currency is particularly important in subject areas where knowledge is rapidly changing, such as the sciences or business.
In other fields currency is important, but it is also useful to look at sources from a broad range of dates, such as religion, history, or the study of literature. Within these topics one often wants viewpoints from contemporary sources as well as sources at various times through to the present.
Every source (including web sites) should include the date it was created - in the case of books there may be later revisions you may want to look for. Articles may expanded upon as a book - or find out if anyone else still cites this article in current articles or books. Web sites are particularly vulnerable in the area of updating - often authors of these sources may put up the site and never do anything else to it, so the data may become inaccurate depending on the subject area being covered or because it contains broken links.
In the case of web sites ask yourself these questions as you evaluate them:
Is there an initial date of completion - or at least information that indicates it has been updated? If there is no date at all it will be hard to determine the age of the information - probably a good indication to avoid that site.
Do the links work? Broken links indicate that there has been no recent maintenance, another sign there could be inaccuracies present.
Is there information for individuals who are responsible for the content? This goes back to the authority issue as well as the currency issus.
Can you determine whether the site is being maintained at all or is it just a forgotten page on a server somewhere?