Technically, those Macs that have a software-based ROM, aka "ROM on RAM" (basically all of the ones that can successfully go to OpenFirmware on your monitor when you press Command-Option-O-F) are New World. Old World Macs have a hardware ROM that fits into a slot inside the computer. While even the earliest PCI Macs did have a very early version of OpenFirmware, they were not complete (on some Macs/clones, OF was viewable only through the serial port if it worked at all). Because of this, they still used the hardware ROM to boot the Mac OS.

Visually speaking, any Macintosh that has a built-in USB port on the logic board is considered a New World Mac.