Replacing Foreign Key with Check Constraint

ALTER TABLE [TABLE NAME}
  ADD CONSTRAINT [NAME OF THE CHECK CONSTRAINT)
    CHECK(dbo.<Name of the Function> (<Key Column Name>) = 1)

CREATE FUNCTION [dbo].[fnCheckTable01]
  (@Table01ID int)
RETURNS bit
AS
BEGIN
  IF EXISTS(SELECT * FROM Table01 (NOLOCK) WHERE Table01ID = @Table01ID)
    RETURN 1
  RETURN 0
END
GO