Checking IFI enabled on SQL server below 2016
According to this MSSQLTips.com article, you can use sys.dm_server_services beginning with SQL 2012 SP4.


SELECT  @@SERVERNAME AS [Server Name] ,
        RIGHT(@@version, LEN(@@version) - 3 - CHARINDEX(' ON ', @@VERSION)) AS [OS Info] ,
        LEFT(@@VERSION, CHARINDEX('-', @@VERSION) - 2) + ' '
        + CAST(SERVERPROPERTY('ProductVersion') AS NVARCHAR(300)) AS [SQL Server Version] ,
        service_account ,
        instant_file_initialization_enabled
FROM    sys.dm_server_services
WHERE   servicename LIKE 'SQL Server (%'



For older versions, you'll need to scrape the log, looking for "Database Instant File Initialization: enabled."