AFAIK, Rquired By is the field you’re looking for. Ran a quick test and I think this was installed from AUR:
~ >>> pacman -Qii libc++
Name : libc++
Version : 6.0.0-1
Description : LLVM C++ standard library.
Architecture : x86_64
URL : https://libcxx.llvm.org/
Licenses : MIT custom:University of Illinois/NCSA Open Source License
Groups : None
Provides : None
Depends On : libc++abi=6.0.0-1
Optional Deps : None
Required By : discord libc++experimental
Optional For : None
Conflicts With : None
Replaces : None
Installed Size : 4,90 MiB
Packager : Unknown Packager
Build Date : P 27 mai 2018 11:32:58 EEST
Install Date : P 27 mai 2018 11:33:09 EEST
Install Reason : Installed as a dependency for another package
Install Script : No
Validated By : None
Backup Files :
And it shows what has caused it to be installed.
Also, Manjaro has ffmpeg package in the official repos and ffmpeg2.8 comes from AUR for sure:
~ >>> yaourt ffmpeg2.8 
1 aur/ffmpeg2.8 2.8.14-1 (3) (2,75)
Complete solution to record, convert and stream audio and video
I hope this helps.
As for the solution to your initial issue, follow bogdancovaciu’s advice for removing and reinstalling the package - it’s the easiest way to go by.
And offtopic - I just love how Manjaro brings some of the frequently used applications from aur to official repos - saves some precious compilation minutes and removed some of the hassle from management.