Tor: How can I verify that an onion address is genuine?

Short answer

Get the address from the organization through a trusted channel, such as its official website or an independently verified announcement, and compare the entire address carefully. A valid-looking 56-character .onion address only proves the service controls that address; it does not prove who operates it. Tor Browser may also show an Onion-Location suggestion when a regular site advertises its onion counterpart.

Sources

Tor Project: Onion services.