Run this query in SSMS or your query tool:
SELECT @@VERSION;
Or check:
SERVERPROPERTY('ProductVersion') AS Version,
SERVERPROPERTY('ProductLevel') AS ServicePack,
SERVERPROPERTY('Edition') AS Edition;
For 12.0.2000.8, you should see ProductLevel = 'RTM'. Microsoft SQL Server 2014 -12.0.2000.8- Key -...
The actual 25-character product key is not stored in plaintext. Instead, a hashed version exists under:
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Microsoft SQL Server\MSSQL12.MSSQLSERVER\Setup\ProductKey
Even if you find this, you cannot reverse it to retrieve the original key. Run this query in SSMS or your query
Microsoft still provides SQL Server 2014 SP3 evaluation media (though not publicly promoted). Install as evaluation, then use your valid key to convert to a licensed edition:
EXEC xp_instance_regwrite N'HKEY_LOCAL_MACHINE', N'SOFTWARE\Microsoft\Microsoft SQL Server\MSSQL12.MSSQLSERVER\Setup', N'SQLServerEdition', REG_SZ, N'Enterprise Edition';
-- Then run: sp_configure 'show advanced', 1; reconfigure;
Better: Use the Setup.exe /Action=EditionUpgrade switch. For 12
If you have lost your product key but legally own a license, here is how to proceed.