As the other commenter said I use a diff tool (I use vimdiff but meld probably works easiest if your not used to vim). I do a pacdiff after every upgrade that will prompt you for all the changed files (most of the times there are none or the changes are minor) and let you compare your version and the .pacnew file. If anything changes in the syntax in a major way (which it almost never does) you will should spot these differences and be able to amend any changes you made in that way.

The example I gave was when some pam config file syntax changed and since I had a custom pam config (because of an encrypted home) it didn’t update the syntax (creating a pacnew file) then I couldn’t login after reboot.

I would recommend doing it manually - you can get a GUI diff tool (I like Meld), run it with both the normal file and pacnew, and merge every change from the pacnew into the main file.