The BeOS demo CD is no longer available from Be, but if you’d like to know whether your system is ready to run BeOS before carving out a partition on your hard drive, just put a BeOS installation CD (either Pro or one you’ve created from Personal Edition) in the drive and boot from floppy or by telling your computer’s BIOS to boot from CD. If you can make it all the way to the first graphical installer screen, you’ll know that your motherboard chipset and video card are basically compatible. To test for sound, network, and other I/O card compatibility, you’ll need to go farther.
From this point, you can manually launch the rest of BeOS. After clicking Agree at the installer screen, hit Ctrl+Alt+Shift+T on the left side of your keyboard to open a Terminal. Type:

/system/Tracker &
/system/Deskbar &

and you’ll be running BeOS. Minimize the Installer – don’t close it, or your system will reboot!
Andreas (korp@bigfoot.com) adds the following:
As an alternative to launching Tracker and Deskbar manually, you can hit Ctrl+Alt+Del to bring up TaskManager, and click „Restart the Desktop“ to have them launched automatically.