How do inspectors know whether a nuclear facility is for peaceful or weapons purposes?

Iran pledged today to allow nuclear inspectors into its newly disclosed uranium enrichment facility near the city of Qom. The Iranians insist the plant was constructed for peaceful purposes; the United States and its allies have accused Iran of pursuing nuclear weapons. How can inspectors tell whether a site is being used for good or evil?  Ploughshares Fund advisor Frank von Hippel and grantee David Albright help Slate's Explainer explain.