Just as scientists proved that the descendants of slave Sally Hemings had also descended from Thomas Jefferson or a close male relation of his, you can compare your chromosomes to those of other folks to whom you may be related. Every man's cheek swab contains a Y chromosome identical to his father's—and his father before him. And anyone's sample contains mitochondrial DNA passed intact from his or her mother. Any two people whose Y chromosomes or mitochondrial DNA match have a forebear in common.
Genetic database companies such as Family Tree DNA perform family or surname "reconstruction projects," comparing DNA sent by people with the same or similar surnames to see if they share the same ancestors. They can also study two clients' genes to help prove they are related to each other.
And they keep the information in a computer database, so that if you like, you'll be put in touch with anyone else in the database who shares a common ancestor with you.
For information about DNA searches, visit www.cyndislist.com/dna.htm, or check out Trace Your Roots With DNA: Using Genetic Tests to Explore Your Family Tree (Rodale, 2004).