Greetings, I do not know from where i got it (could have been the wiki or forums or reddit, but most likely is wiki), but i have always done `pacman -Qttdq`. I do no think it will make a huge difference though.
You can mark packages to be installed explicitly by `pacman -D --asexplicit`. You can also uninstall it and re install with pacman -S, and it should work, though for some packages you can not remove them (either have too many dependants or really required for system or pacman (like bash)). If you truly want to experiment with removing them, try `pacman -Rdd` but please do not use it if you can, especially with any dependencies of pacman, as that is a very fast way to break pacman.
