declare @test table (Col1 nvarchar(500));

declare @testdata nvarchar(500);
set @testdata = '';

-- make a piece of sample data with all characters from ascii 1-128
;WITH
TENS      (N) AS (SELECT 0 UNION ALL SELECT 0 UNION ALL SELECT 0 UNION ALL 
                  SELECT 0 UNION ALL SELECT 0 UNION ALL SELECT 0 UNION ALL 
                  SELECT 0 UNION ALL SELECT 0 UNION ALL SELECT 0 UNION ALL SELECT 0),
THOUSANDS (N) AS (SELECT 1 FROM TENS t1 CROSS JOIN TENS t2 CROSS JOIN TENS t3),
MILLIONS  (N) AS (SELECT 1 FROM THOUSANDS t1 CROSS JOIN THOUSANDS t2),
TALLY     (N) AS (SELECT ROW_NUMBER() OVER (ORDER BY (SELECT 0)) FROM MILLIONS)
SELECT TOP (128) @testdata = @testdata + char(N)
  FROM TALLY 
 ORDER BY N
insert into @test values (@testdata)

-- show the sample data to show some of the junk characters
select * from @test

-- you can make this part into a function
;WITH
TENS      (N) AS (SELECT 0 UNION ALL SELECT 0 UNION ALL SELECT 0 UNION ALL 
                  SELECT 0 UNION ALL SELECT 0 UNION ALL SELECT 0 UNION ALL 
                  SELECT 0 UNION ALL SELECT 0 UNION ALL SELECT 0 UNION ALL SELECT 0),
THOUSANDS (N) AS (SELECT 1 FROM TENS t1 CROSS JOIN TENS t2 CROSS JOIN TENS t3),
MILLIONS  (N) AS (SELECT 1 FROM THOUSANDS t1 CROSS JOIN THOUSANDS t2),
TALLY     (N) AS (SELECT ROW_NUMBER() OVER (ORDER BY (SELECT 0)) FROM MILLIONS)
SELECT N, [Character] = SUBSTRING(Col1, N, 1)
  FROM TALLY, @test
 WHERE N < len(Col1)
   AND ascii(SUBSTRING(Col1, N, 1)) NOT BETWEEN 32 and 126