Here’s a quickie for those who use OS X’s Mail app.
Want to instantly see the reply you’ve sent to a mail? Just click the small arrow to the left of the mail in the inbox list (see the screenshot below where it’s highlighted — it’s pretty small but far from invisible).
If there’s more than one reply then you’ll be shown the most recent you’ve sent.
This also works if you want to view forwarded emails you’ve sent. Instead of the curled arrow indicating a reply, you should click the small right-facing arrow next to the original email.
If you want to see a listing of all emails received in that particular thread, click the small number to the right, beneath the time and date.