When I type a name into the "To:" box a couple of things happen to let me know if someone has iMessage:
1) There is a blue icon on the far right of their name
2) The send button turns blue when I've entered it
(otherwise green)
I believe this will show you if they have iMessage even if you haven't texted them yet.
iMessage is also be displayed in the text (compose) box, but is "grayed out" until you start typing. If they don't have iMessage, than the text box will read "text message".