Using Apple's new app Messages beta you can discover who you can send Messages to.
In Messages, press Cmd+N to open a blank message window. In the 'To:' field, which is highlighted by default, begin typing a name. A list will appear showing all the possible completions. Once you have typed one unique name, the list will show all the addresses known for that person, and will put a blue chat bubble next to any item that you can send a Message to. This works for both e-mail addresses and phone numbers.
[crarko adds: I don't have the beta handy so I haven't tested this one.]
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