Here is the way to find SQL Server CD key, which was used to install it on machine. If user do not have permission on the SP, please login using SA username. Expended stored procedure xp_regread can read any registry values. I have used this XP to read CD_KEY. This is undocumented Stroed Procedure and may not be supported in Future Version of SQL Server.
EXEC xp_regread 'HKEY_LOCAL_MACHINE','SOFTWARE\Microsoft\Microsoft
Reference : Pinal Dave (https://blog.sqlauthority.com)