You don't have to do all the work by yourself. The organizations listed here can help you determine whether a charity is legit—and whether it is operating efficiently.
Independent Charities of America (http://www.independentcharities.org)
One of the most respected charity-rating organizations in the country, this group awards the Independent Charities Seal of Excellence to member organizations that meet the group's highest standards. Of the 1 million charities operating in the United States today, fewer than two thousand have been awarded this seal. Independent Charities of America
Charity Navigator (http://www.charitynavigator.org)
Charity Navigator provides information on more than five thousand charities, evaluating the financial health of each group it rates. The ratings are based on organizational efficiency and organizational capacity. If a charity spends less than a third of its budget on the programs and services it exists to provide, then Charity Navigator gives that charity a score of zero for organizational efficiency. Under "Tips" on this website you'll also find a very useful list: "The Top 10 Practices of Savvy Donors."
BBB Wise Giving Alliance (http://www.bbb.org/us/charity)
The BBB Wise Giving Alliance awards the BBB national charity seal to groups that meet its standards. Don't expect small, local charities to have a seal; only national charities are eligible. Among other requirements, the BBB expects charities to spend at least 65 percent of their total income on program activities and to spend no more than 35 percent of contributions on fund- raising. It also expects charities to avoid accumulating funds that could be used for current activities.