I don't think there's an automatic way. Packages installed through build-dep are recorded in the same way as if they'd been directly requested by name.

If you know approximately when you ran aptitude build-dep, you can find which packages got installed when you ran the command by looking through the logs in /var/log/aptitude* or /var/log/apt/*.

You can run apt-cache showsrc PACKAGENAME | grep '^Build-depends:' to list the build dependencies of the package. Review each of them to see if you want them; mark the ones you don't care about as not manually installed (apt-get markauto PACKAGENAME) and flush the non-required packages (apt-get autoremove).