Try volunteering for one of its programs or tour its operations. Contact the charity and request its annual report and IRS Form 990. Every legitimate charity will be happy to comply with your request. They WANT to build a relationship with you. Go to GuideStar to view a charity's IRS Form 990.
Different charities attack the same problem from different angles.
Your desire to fight cancer may coincide with the activities of one organization more than others. Visit a charity's own web site to find out more about its unique mission and programs. Altruism and a desire to help are the best reasons for giving. Do not be swayed by charities asking for your dollar through guilt and intimidation. Do not give to an unknown charity over the telephone, especially if it pressures you to give immediately, or refuses to send information unless you make a pledge.