I've gotten to the point where I do not install any AUR helpers on my systems, and manually download PKGBUILDs and install with makepkg. These extra steps force me to 1) review the PKGBUILD + *.install files, and 2) make me reconsider whether or not I want to go through the effort for a package (i.e. "do I really want this thing")
If you want to see all software installed from outside repos defined in /etc/pacman.conf, you can use this pacman option:
pacman -Qm
It's always a good idea to periodically review this list as well.