If you're spending time this weekend making charitable contributions online or mailing checks by Dec. 31 to get a 2012 tax deduction, you can check a charity-rating service to see how wisely the charity is spending your money.

Sounds like a good idea but it's not as easy as it sounds. Only about 7,500 of the 1 million charities in the U.S. are evaluated by three popular review agencies such as Charity Watch, Charity Navigator and the Better Business Bureau's Wise Giving Alliance, said Ken Berger, CEO of Charity Navigator.

Making matters more complicated, the ratings, which can be a letter grade, one to four stars or a simple "yes" or "no" seal of accreditation depending on the source, vary widely among the reviewers. The Humane Society of the United States, for example, is given a "D" rating from Charity Watch, 4 stars from Charity Navigator and the BBB Accreditation Seal.

Conflicting ratings means more work for conscientious givers, and may make some givers avoid the ratings all together. But if you're giving to a charity for the first time, it can still be helpful to check some of the ratings at Charity Watch, Charity Navigator, Give Well or the BBB's Wise Giving Alliance.

If a charity isn't listed by any ratings services, Charity Navigator offers some guidelines for performing your own due diligence.