Sorry for the confusion -- this whole thing is about how passwords are transmitted, not how they are stored.

When it comes to password storage, though, the only way to find this out is: (1) if you are able to look at the backend systems, (2) if someone hacks in, as happened with rockyou (google: rockyou passwords), or (3) if they send you your password back in the clear in a password reminder email, rather than forcing you to create a new one. Note that #2 and #3, if they happen, only indicate that passwords are stored in the clear. If they don't happen that doesn't mean they are not stored in the clear.