There are plenty of tools out there to help you with email management, but if you want to do it yourself, blogger Jonathan Kim has created a Google Script that automatically shows you which emails you have sent that haven't received replies.
Once the script is installed it scrubs through your email and find messages between five and 14 days old where you were the last person to respond. This essentially shows you any emails that you've sent out that you haven't heard back on.
A service like Boomerang can also do this, but the script is handy because it keeps everything on Gmail and doesn't use any third-party services. Plus, you can tweak the script so it does exactly what you want. Head over to Kim's site for a copy of the script as well as installation instructions.
Find Emails with No Reply [Jonathan Kim]