The way to find packages installed from AUR, is to search for "foreign" packages. man pacman says:
-m, --foreign
Restrict or filter output to packages that were not found
in the sync database(s). Typically these are packages
that were downloaded manually and installed with --up‐
grade.
So that means performing a search like this will in practice list all packages from AUR, plus any other packages you may have created installed manually. For me, and I expect most users, these will all be from AUR:
# pacman -Qm
Happy searching!