How to recognise that a constraint is unnamed in Sql Server?

I could not find any dedicated designation for unnamed constraints in Sql Server

It is there. You can use the below

WITH T
     AS (SELECT is_system_named, name, type_desc
         FROM   sys.check_constraints
         UNION ALL
         SELECT is_system_named, name, type_desc
         FROM   sys.default_constraints
         UNION ALL
         SELECT is_system_named, name, type_desc
         FROM   sys.key_constraints
         UNION ALL
         SELECT is_system_named, name, type_desc
         FROM   sys.foreign_keys)
SELECT name,
       type_desc
FROM   T
WHERE  is_system_named = 'true'