You could do something like "slackpkg install x/*" which should install it.
To check if it's installed, just do something like "slackpkg search xorg-server" or something and it'll list the matches with that name and tell you if it's installed or not.
You can also just do:
ls /var/log/packages | grep xorg-server
Also, a full install is usually recommended unless you know what you're doing (and it appears you may have possibly left out the [X] package series upon installing).