SQL Serverの制約が存在するかどうかを確認するにはどうすればよいですか？

 SELECT
    * 
    FROM INFORMATION_SCHEMA.REFERENTIAL_CONSTRAINTS 

または、これを試してください

  SELECT OBJECT_NAME(OBJECT_ID) AS NameofConstraint,
SCHEMA_NAME(schema_id) AS SchemaName,
OBJECT_NAME(parent_object_id) AS TableName,
type_desc AS ConstraintType
FROM sys.objects
WHERE type_desc LIKE '%CONSTRAINT' 

または

IF EXISTS(SELECT 1 FROM sys.foreign_keys WHERE parent_object_id = OBJECT_ID(N'dbo.TableName'))
 BEGIN 
ALTER TABLE TableName DROP CONSTRAINT CONSTRAINTNAME 
END