The signed list and the captcha check.
Two checks confirm a genuine WTN address, and the verification cues show in English and French.
The real onion is embedded in the login captcha image and reprinted in the page header. Compare either against your bar before your password. A clone that copies the WTN theme struggles to fake both at once.
WTN signs its mirror list with a PGP key. Verifying the signature proves an address came from the market. The addresses on this page are checked against that signed list.