If you have several licenses and do not know which is activated on the selected iMac, you need to click “Apple” sign in the top left corner of the screen and select “About This Mac”
Then click “System Report…”
You will see “Hardware UUID”
This is your computer’s Hardware ID (GUID). Our online activation system uses this value to bind licenses to your computer.
If the license is activated, the device’s Hardware ID will be shown in GUID field of Product Details. In this example, it’s the same as Hardware ID of the computer, which means it’s activated on that computer.