checking, and finding the source for dependencies
 

I'm looking to install, and perhaps create a SlackBuild) for an application that I think might be interesting.

The dependencies listed are:

libx11-dev, libxt-dev, libimlib2-dev, giblib-dev, and libxinerama-dev

I've checked to see if the packages are installed w/:

I've also looked for any existing SlackBuilds, but the only one that showed up in the database is: "giblib"

Where can I look to find these dependencies, their sources, or determine whether they're already installed in Slackware64 13.1?

Also, how would one determine if this can compile for a 64bit, non multi-lib install?

Slackware doesnt make split dev packages. These are all in Slackware besides giblib.
You can find giblib at slackbuilds.org

Other distributions often waste a lot of time and effort by splitting upstream packages into a core package and what they call a "development" package that contains the header files, library documentation, or whatever.

In a world of 1Tb disks, this is pretty futile. Slackware doesn't do it. Slackware's libX11 package has everything you will need. Same for libXt and libXinerama, and the giblib and imlib2 packages from SBo. (You'll need imlib2 from SBo - Slackware itself only ships the older imlib.)

By the way, your command "ls /var/log/packages |grep libx11" is in general a useful trick, but in this case it won't tell you what you seek, because the library is libX11 (upper case X, not lower case). And that's the fault of the developers of your upstream package, who misled you when they lazily just listed a bunch of Debian-style dependencies. When you're packaging, sometimes it pays to be smarter and more paranoid than the upstream devs.

Edit: Forgot to answer your second question. How would one determine if this can compile for a 64bit, non multi-lib install? Just Do It. If you've got a multilib setup, create a non-multilib virtual machine to test it.

Errm sorry imlib2 should be obtained from slackbuilds.org as well like 55020 says