This question came close to what I was looking for, but missed the mark.
After some adventures trying to get device names as displayed from xrandr into xorg.conf I began to wish that there was an X11 tool that could enumerate all the display devices it saw on the system like xrandr does and also allow you to find out what PCI bus they were on so that you were certain of what the hardware source was for each one.
As it stands, you have to use different tools to gain that information or slog through the Xorg log and piece together info from multiple lines to associate an output display or screen name with a PCI bus.
Certainly one could run startx as a whole then look at the Xorg log (/var/log/Xorg.0.log), but that is unwieldy when what you are trying to do is probe that info in advance of actually trying to launch X to put into your Xorg.conf. One could wish for a --dry-run option or something for Xorg that probes the PCI bus for GPUs and dumps it to a screen instead of a log. Basically, an easy to use commandline tool is what I'm wishing for.
Sadly, it appears that the /bin/X or /bin/Xorg utility back in 2005 had two nifty switches which appear in its older man pages:
-probeonly Causes the server to exit after the device probing stage. The xorg.conf(5x) file is still used when this option is given, so information that can be auto-detected should be commented out.
-scanpci When this option is specified, the Xorg server scans the PCI bus, and prints out some information about each device that was detected. See also scanpci(1) and pcitweak(1) . source
This would be perfect, but alas it is gone.
What alternatives are there other than lspci + reviewing Xorg logs?