There are a number of nonpartisan websites that can assist in determining what's true, what's misleading, and what's just plain wrong in both the media's election coverage and the candidates' own ads.
FactCheck.org is a project of the Annenberg Public Policy Center of the University of Pennsylvania. They do fairly in-depth analyses, explaining fully why something is misleading, and frequently provide audio and video links to the content being checked.
PolitiFact is a service of the St. Petersburg Times and Congressional Quarterly Inc. This site features a "truth-o-meter," a graphic that rates the truthfulness of the ad or claim. While usually not as in-depth as FactCheck, the content on this site is well-organized and accessible in a variety of ways (you can browse by candidate or subject, among other things).
The Center for Public Integrity is a "nonprofit, nonpartisan, non-advocacy, independent journalism organization" that uses examines political and campaign issues in depth. These folks also have a Buying of the President site which looks at how money influences presidential campaigns.
The Fact Checker is a blog from the Washington Post that analyzes campaign statements in a similar way to FactCheck.org and PolitiFact, prompted by reader suggestions. Submit a question there if you're wondering about a claim that isn't being addressed on the other sites.