You can use pacman -S to just install what you want. It will not put you in a partial update state, unless certain commands are run. The most notable is -Sy which only updates the local database (another is -Syuw as it updates the local database, but only downloads the updates, it doesn’t install them).
The filename contains the version and pacman requests the filename, so if you get a 404 (file not found) error, then the package has been updated and you need to update to install it.
If you update the local database using -Sy or -Syuw and then install an updated package without updating the system then you’re in a partial update state. Which may or may not cause problems, but is unsupportable (other than by telling you to update).
If you only use -S and -Syu then you can’t get into a partial update state, unless there’s a failed update that isn’t corrected, or a packaging issue.
This only applies to the repos (and pacman), the AUR and manual installs can result in mismatched versions.
EDIT:
Corrected an omission, about failed updates.