Q: iMac Stolen

The following has some methods to find the serial number Find the serial number of your Apple hardware product - Apple Support but if you don't have the invoice or packaging then the other possibility is if you entered it to your support profile: https://supportprofile.apple.com/

If you registered your iMac at My SupportProfile  (https://supportprofile.apple.com/), sign in with your AppleID and look at your registered products. You will see your serial number there.

Otherwise try to find the receipt for for MAc. It will show the serial number as well.

.  Will Apple know to notify me if someone tries to use it?

NO. Apple is not a police investigative agency and has no way of knowing who the rightfull owner is.