Model
How do you find the model on the imac itself?
"Sub-model" is probably what you want. If the computer works, you simply do "About this Mac..." from your Apple menu at the left end of the menubar:
If the computer does not work, you look on the botom of the "foot" of the stand for the serial number and enter it here:
Check Your Service and Support Coverage - Apple Support
The return will show the sub-model: