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The built-in checksum in every Torzon link
Onion addresses carry their own error detection, with one important limit.
What the checksum does
Two bytes of the address are calculated from the key and version. Tor Browser rejects an address whose checksum does not match.
Typos fail safely
If you mistype one character of a torzon link, the checksum almost always breaks and the address simply does not open.
Lookalikes still pass
An attacker who generates a completely new key gets a new, valid checksum. A deliberate fake is a valid address, just for the wrong service.
The takeaway
An address that opens is only proof that it is well formed, not that it belongs to Torzon.
The checksum catches typos, not impostors; a working Torzon link still has to be compared with a checked source.