Prairie Group's Support
Determining if Classic (OS 9) is already installed.
Double-click on your OS X hard drive icon to open it. You will see a folder named "System" which is your OS X operating system. If you have Classic installed, you will also see a folder named "System Folder" which is your OS 9 operating system. Note that the OS 9 operating system actually has the word "Folder" as part of the folder name.
Installing Classic on an OS X Computer that is missing OS 9 (Classic)
Note: Classic can be installed on all OS X computers except the Intel-based computers released in January 2006 and any running Mac OS X 10.5, Leopard.
1) Original OS X computers came with Classic pre-installed. If you need to re-install OS 9, see the section at the bottom of this page.
2) Later Apple stopped pre-installing Classic, but included a CD labeled Classic.
3) Still later Apple included Classic on Install Disc 2 as a part of the overall OS X installation discs. The following is from page 56 of an iMac user guide.
Re-installing Classic on an OS X Computer that has a damaged OS 9 (Classic)