Check the bottom of the Chromebook for a label with the serial number
Open the Chromebook and look under the battery if it is removable
Press Alt + Shift + I to open the ChromeOS diagnostics or device info screen if available
Open Settings
Select About ChromeOS
Select Additional details or Device details
Look for Serial number in the device information
Check the original box or packaging
Check the purchase receipt or invoice
Sign in to your Google Admin console if the device is managed
Check the Chromebook recovery utility or device management records if available