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.
>...if they send you your password back in the clear in a password reminder email, rather than forcing you to create a new one.
This only indicates they store the password in a recoverable format. It could be encrypted. It shouldn't be stored encrypted -- hashing is the way to go -- but it doesn't mean the server has "hunter2" sitting on disk somewhere.
It is extremely annoying to make "recoverable" encryption schemes even nominally secure. Most of the time, anything that can mail your password might as well be storing the raw passwords. It's bad mojo to suggest that "reversable encryption" is a good strategy.
Every time this comes up, someone suggests some rube goldberg contraption involving multiple hosts and escrowed keys which is hard to refute except that it never exists in the real world and, even if it did, would still only provide security that asymptotically approaches hashing.
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.