When config files change with an update pacman writes the new one as a .pacnew file instead of overwriting the existing one. That's always an issue with updates on config files (although one less severe on slow updating distros). Do you keep the old config to keep your changes intact while ignoring new default options that should be in there or do you replace the old config with a recent one but risk using manual edits?

pacdiff basically just searches for .pacnew files and then queries you what to do with each.

You can define a viewer via a DIFFPROG= variable (I simply use vim -d because that's already my editor of choice but the package pulls diffutils as a dependency so just diff works, too) and compare the new default config file with the existing one side-by-side (with differences highlighted), so you can either copy your personal edits to the new one or copy additional or changed default options to your old one, whatever is easier. After that you have the option to dismiss/delete the .pacnew file or rename it to replace the old one.

PS: pacdiff is part of the pacman-contrib package. That package also provides checkupdates.

Running pacman -Syu updates your package list to the recent one, then updates all your outdated packages. You are not actually supposed to update the list, then abort the update of packages or just update some of them because Archlinux always assumes up-to-date systems (see: unsupported partial upgrades). So pacman should not be used to just check for available updates without updating. That's where checkupdates comes in, simulating pacman -Syu without actually touching your system and just getting a list of available updates.

PPS: the AUR has a similiar aur-check-updates tools to do the same for AUR packages.