The first question — whether you have replied to an email — is answered by the small leftward-swung arrow in the message list in Mac OS X Mail.
But what about the second question — what exactly you wrote in your reply? Do you have to dig and search the Sent folder? No, the small arrow has the answer to this question, too.
Find Your Reply to an Email Fast in Mac OS X Mail
To find your reply to an email fast in Mac OS X Mail:
- Click on the leftward-pointing circled arrow in the Flags column of the message.
Mac OS X Mail automatically opens your reply in a new message window. If you have answered an email more than once, only the last answer will open.

