List of all database without a specific index
CREATE TABLE #dbs(db sysname);

DECLARE @cmd nvarchar(max) = N'INSERT #dbs SELECT ''?''
  WHERE EXISTS (SELECT 1 FROM ?.sys.tables WHERE name = N''TableA'')
  AND NOT EXISTS (SELECT 1 FROM ?.sys.indexes AS i
    INNER JOIN ?.sys.tables AS t
    ON i.[object_id] = t.[object_id]
    WHERE t.name = N''TableA''
      AND i.name = N''IX_Missing_Index'');';

EXEC master.dbo.sp_foreachdb @command = @cmd, @user_only = 1;

SELECT db FROM #dbs;

DROP TABLE #dbs;


You can get sp_foreachdb here (it is also being open sourced and added to Brent Ozar Unlimited's SQL Server First Responder Kit).