Knowing if You Replied to a Message

In Google Mail, Conversation View helps you see how a message and it's replies have evolved over time.

Understanding Conversation View in Google Mail

With Conversation View on

Look at a message in your inbox, for example. If you replied to the message, in the sender field you will see the original sender's name followed by "me" and (2).
In this example, I received a message from Nichole and then replied, and the Nichole, me  2 shows that I did in fact reply:

Conversation View off

You'll need to look in the Sent Mail

 Learn more about conversation view.