Request an official transcript from the candidate, sent directly from the his school. Official transcripts are sealed, signed and stamped by the registrar's office. The individual cannot tamper with it because the documents do not go through them.
Call the registrar office directly. Under the Family Educational Rights and Privacy Act (FERPA), colleges are not allowed to release GPA information and academic records over the phone, but they can confirm graduate dates and major/degree type.
Request the person show you the original degree or diploma. As an employer, asking to prove a bachelor's degree can eliminate or deter some applicants. However, degrees can be falsified, or acquired from non-accredited colleges.