Following script are very useful to know all the constraint in the database. I use this many times to check the foreign key and primary key constraint in database. This is simple but useful script from my personal archive.
SELECT OBJECT_NAME(OBJECT_ID) AS NameofConstraint,
SCHEMA_NAME(schema_id) AS SchemaName,
OBJECT_NAME(parent_object_id) AS TableName,
type_desc AS ConstraintType
WHERE type_desc LIKE '%CONSTRAINT'
Reference : Pinal Dave (http://blog.SQLAuthority.com)