The only way how a website can prove it's NOT storing your password in plaintext is to never receive it in plaintext. This can be only achieved via client-side hashing script. Unless you can analyze website code and/or sniff traffic you have to assume that the admin has full knowledge of your password in plaintext.
See thisthis question from an admin who wanted to give his users peace of mind you seek. it has many interesting answers, albeit most of them focus on security instead of proof-of-not-storing-plaintext.