--========================--
--== CREATE SAMPLE DATA ==--
--========================--
SELECT yourData
INTO #yourTable
FROM (VALUES('MNN.helloworld'),('GFF.goodbyeworld'),('SSW.seeyoulaterworld.oops')
      )a(yourData);
      
--========================--
--==      SOLUTION      ==--
--========================--
SELECT *
FROM #yourTable
WHERE LEN(yourData)-1 = LEN(REPLACE(yourData COLLATE Latin1_General_BIN2,'.',''));

SELECT *
FROM #yourTable
WHERE LEN(CAST(yourData AS NVARCHAR(MAX)))-1 = LEN(REPLACE(CAST(yourData AS NVARCHAR(MAX)) COLLATE Latin1_General_BIN2,'.',''));

declare @temp table(t varchar(50))
insert into @temp
select * from
(
values
('a.p'),
('y.a.p'),
('k.p')
)a (name)

select *
from @temp
where CHARINDEX('.',t,CHARINDEX('.',t)+1)=0

SET NOCOUNT ON;
IF object_id('tempdb..#testEnvironment') IS NOT NULL
BEGIN
    DROP TABLE #testEnvironment;
END;
  
--1,000,000 Random rows of data
SELECT IDENTITY(INT,1,1) AS ID, CAST(LEFT(yourData, LEN(yourData) - 1) AS NTEXT) AS yourData
INTO #testEnvironment
FROM (SELECT TOP 1000000 
      REPLICATE(
      REPLACE(REPLACE(REPLACE(REPLACE(REPLACE(REPLACE(REPLACE(REPLACE(REPLACE(REPLACE(
      REPLACE(NEWID(),'-','')
      ,'0',''),'9',''),'8',''),'7',''),'6',''),'5',''),'4',''),'3',''),'2',''),'1','') + '.',
      (ABS(CHECKSUM(NEWID())) % 2) + 2)
      FROM master.dbo.syscolumns sc1, master.dbo.syscolumns sc2, master.dbo.syscolumns sc3
      )a(yourData);

DECLARE @HOLDER INT, @Duration CHAR(12), @StartTime DATETIME;

SELECT @StartTime = GETDATE();

SELECT @HOLDER = ID
FROM #testEnvironment
WHERE CHARINDEX('.', yourData, CHARINDEX('.', yourData) + 1) = 0;

SELECT @Duration = CONVERT(CHAR(12),GETDATE()-@StartTime,114);
RAISERROR('Duration for CHARINDEX = %s',0,1,@Duration) WITH NOWAIT;

SELECT @StartTime = SYSDATETIME();

SELECT @HOLDER = ID
FROM #testEnvironment
WHERE LEN(CAST(yourData AS NVARCHAR(MAX)))-1 = LEN(REPLACE(CAST(yourData AS NVARCHAR(MAX)) COLLATE Latin1_General_BIN2,'.',''));

SELECT @Duration = CONVERT(CHAR(12),GETDATE()-@StartTime,114);
RAISERROR('Duration for LEN = %s',0,1,@Duration) WITH NOWAIT;

Duration for CHARINDEX = 00:00:10:660
Duration for LEN = 00:00:06:327

DECLARE @HOLDER INT, @Duration CHAR(12), @StartTime DATETIME;

SELECT @StartTime = SYSDATETIME();

SELECT @HOLDER = ID
FROM #testEnvironment
WHERE LEN(CAST(yourData AS NVARCHAR(MAX)))-1 = LEN(REPLACE(CAST(yourData AS NVARCHAR(MAX)) COLLATE Latin1_General_BIN2,'.',''));

SELECT @Duration = CONVERT(CHAR(12),GETDATE()-@StartTime,114);
RAISERROR('Duration for LEN = %s',0,1,@Duration) WITH NOWAIT;

SELECT @StartTime = GETDATE();

SELECT @HOLDER = ID
FROM #testEnvironment
WHERE CHARINDEX('.', yourData, CHARINDEX('.', yourData) + 1) = 0;

SELECT @Duration = CONVERT(CHAR(12),GETDATE()-@StartTime,114);
RAISERROR('Duration for CHARINDEX = %s',0,1,@Duration) WITH NOWAIT;

Duration for LEN = 00:00:10:927
Duration for CHARINDEX = 00:00:05:900

SELECT @HOLDER = ID
FROM #testEnvironment
WHERE CHARINDEX('.', yourData COLLATE Latin1_General_BIN2 , CHARINDEX('.', yourData COLLATE Latin1_General_BIN2) + 1) = 0;

SET NOCOUNT ON;
IF object_id('tempdb..#testEnvironment') IS NOT NULL
BEGIN
    DROP TABLE #testEnvironment;
END;
  
--1,000,000 Random rows of data
SELECT IDENTITY(INT,1,1) AS ID, CAST(LEFT(yourData, LEN(yourData) - 1) AS NTEXT) AS yourData
INTO #testEnvironment
FROM (SELECT TOP 1000000 
      REPLICATE(
      REPLACE(REPLACE(REPLACE(REPLACE(REPLACE(REPLACE(REPLACE(REPLACE(REPLACE(REPLACE(
      REPLACE(NEWID(),'-','')
      ,'0',''),'9',''),'8',''),'7',''),'6',''),'5',''),'4',''),'3',''),'2',''),'1','') + '.',
      (ABS(CHECKSUM(NEWID())) % 2) + 2)
      FROM master.dbo.syscolumns sc1, master.dbo.syscolumns sc2, master.dbo.syscolumns sc3
      )a(yourData);

DECLARE @HOLDER INT, @Duration CHAR(12), @StartTime DATETIME;

DBCC FREEPROCCACHE;
DBCC DROPCLEANBUFFERS;

SELECT @StartTime = GETDATE();

SELECT @HOLDER = ID
FROM (SELECT ID, yourData, MAX(rn)
      FROM (SELECT ID, yourData, split.Part.value('text()[1]', 'VARCHAR(MAX)'), ROW_NUMBER() OVER(PARTITION BY yourData ORDER BY (SELECT NULL))
            FROM (SELECT CAST('<p>' + REPLACE(CAST(yourData AS VARCHAR(MAX)) COLLATE Latin1_General_BIN2,'.','</p><p>') + '</p>' AS XML), 
                  CAST(yourData AS VARCHAR(MAX)), ID
                  FROM #testEnvironment) innerQ(xmlField, yourData, ID) 
            CROSS APPLY innerQ.xmlField.nodes('p') split(Part)
            ) a(ID, yourData,splitData,rn)
      GROUP BY ID, yourData
      ) a(ID, yourData, rn)
WHERE rn = 2;

SELECT @Duration = CONVERT(CHAR(12),GETDATE()-@StartTime,114);
RAISERROR('Duration for ugly xml split = %s',0,1,@Duration) WITH NOWAIT;

DBCC FREEPROCCACHE;
DBCC DROPCLEANBUFFERS;

SELECT @StartTime = GETDATE();

SELECT @HOLDER = ID
FROM #testEnvironment
WHERE CHARINDEX('.', yourData, CHARINDEX('.', yourData) + 1) = 0;

SELECT @Duration = CONVERT(CHAR(12),GETDATE()-@StartTime,114);
RAISERROR('Duration for CHARINDEX = %s',0,1,@Duration) WITH NOWAIT;

DBCC FREEPROCCACHE;
DBCC DROPCLEANBUFFERS;

SELECT @StartTime = GETDATE();

SELECT @HOLDER = ID
FROM #testEnvironment
WHERE LEN(CAST(yourData AS NVARCHAR(MAX)))-1 = LEN(REPLACE(CAST(yourData AS NVARCHAR(MAX)) COLLATE Latin1_General_BIN2,'.',''));

SELECT @Duration = CONVERT(CHAR(12),GETDATE()-@StartTime,114);
RAISERROR('Duration for LEN = %s',0,1,@Duration) WITH NOWAIT;

DBCC FREEPROCCACHE;
DBCC DROPCLEANBUFFERS;

SELECT @StartTime = GETDATE();

SELECT @HOLDER = ID
FROM #testEnvironment
WHERE CHARINDEX('.', yourData COLLATE Latin1_General_BIN2 , CHARINDEX('.', yourData COLLATE Latin1_General_BIN2) + 1) = 0;

SELECT @Duration = CONVERT(CHAR(12),GETDATE()-@StartTime,114);
RAISERROR('Duration for CHARINDEX with COLLATE = %s',0,1,@Duration) WITH NOWAIT;

DBCC execution completed. If DBCC printed error messages, contact your system administrator.
DBCC execution completed. If DBCC printed error messages, contact your system administrator.
Duration for ugly xml split = 00:00:48:980
DBCC execution completed. If DBCC printed error messages, contact your system administrator.
DBCC execution completed. If DBCC printed error messages, contact your system administrator.
Duration for CHARINDEX = 00:00:08:640
DBCC execution completed. If DBCC printed error messages, contact your system administrator.
DBCC execution completed. If DBCC printed error messages, contact your system administrator.
Duration for LEN = 00:00:09:087
DBCC execution completed. If DBCC printed error messages, contact your system administrator.
DBCC execution completed. If DBCC printed error messages, contact your system administrator.
Duration for CHARINDEX with COLLATE = 00:00:05:323