A "boot CD" for a Mac is the Mac OS X installation disc that comes with it. You obviously don't have that... Mac's should always be sold with its original discs, but that's a pipe dream, I suppose.
The next best thing is a retail Mac OS X installation disc. These are the discs Apple sells "in a box" to owners of existing Macs in order to upgrade to the latest OS release. If you get one for Panther (10.3) or Tiger (10.4), startup from it by holding down the C key. When you get to the first Installer screen, go to the Apple menu and select +About This Mac+. That will tell you processor type, processor speed, and amount of RAM installed.
Make sure it's a
retail disc. They are black with a silver X. Mac OS X installation discs that came with a particular Mac model (usually gray color) will usually not work on other models.
The current OS release, Leopard (10.5), would not be a good choice for this purpose because many G4 and below Macs cannot run it.
Tiger will work on all PowerPC Macs that originally had a G4 and have at least 256mb of RAM. It will also work on slot-loader G3 iMac (not tray-loaders), Blue and White Power Mac G3 (not beige), PowerBook G3 (only the model with FireWire) - not earlier G3 models.
Panther will work on all PowerPC Macs that originally had a G4 and most G3's that have at least 128mb of RAM, so it may be the best choice for this purpose.
There are some G3 Macs that need to have a firmware upgrade before running Mac OS X (even the installer) for the first time. This is particularly harmful to slot-loading G3 iMacs.
http://support.apple.com/kb/HT1395
If most of the Macs have a FireWire port, and you have an external FireWire drive, you should install either Panther or Tiger on that drive. Then you can boot each Mac from that external FireWire drive and run System Profiler to get more detailed information about the system. Mac OS X installations are generalized, so as long as a PowerPC Mac ran the installation, it should work with supported PowerPC Macs. Some Macs do not have FireWire. USB external drives are not bootable on most PowerPC Macs.
