Yes, people searches are completely legal in the United States. Following the guidelines outlined by the Fair Credit Reporting Act (FCRA) will help you avoid running into any legal problems. Still, as long as you don't do anything illegal, there's no reason for you to get in trouble.
To avoid any problems, avoid running searches on people who are applying for a job, current employees, domestic helpers, and people who have court orders against you. Otherwise, search away!