for x in $(pacman -Qqm); do packer -Ssq --auronly $x | egrep '^'"$x"'$'; done
Breaking it down:
for x in $(...)
Use the output of the $(…) as input for the loop
List all locally installed, foreign packages quietly
Start of the loop – Only include the package name
do packer -Ssq --auronly $x
Pull package matches from AUR quietly
| egrep '^'"$x"'$'
Grep for lines including ONLY the actual package name, otherwise you get partial matches, like accountsservice would match aur/accountsservice-ubuntu instead of extra/accountsservice where it came from. The regex says match ^ start of line, the variable $x (package name), then $ end of line.