How do I validate all my constraints in postgresql?

Well, if you need a way to check if all the foreign keys in your table are valid, this might help ( it just validates all foreign keys in your schema )

do $$
  declare r record;
BEGIN 
FOR r IN  (
  SELECT FORMAT(
    'ALTER TABLE %I VALIDATE CONSTRAINT %I;',
    tc.table_name,
    tc.constraint_name
  ) AS x
  FROM information_schema.table_constraints AS tc  
  JOIN information_schema.tables t ON t.table_name = tc.table_name and t.table_type = 'BASE TABLE' 
  JOIN information_schema.key_column_usage AS kcu ON tc.constraint_name = kcu.constraint_name 
  JOIN information_schema.constraint_column_usage AS ccu ON ccu.constraint_name = tc.constraint_name 
  WHERE  constraint_type = 'FOREIGN KEY' 
    AND tc.constraint_schema = 'public'
)
  LOOP
    EXECUTE (r.x);  
  END LOOP;
END;
$$;

The solution proposed By Evan Carroll was not working for me.

I had to adapt it to mark before all constraints as not valid.

do $$
  declare r record;
BEGIN
FOR r IN  (
  SELECT FORMAT(
    'UPDATE pg_constraint SET convalidated=false WHERE conname = ''%I''; ALTER TABLE %I VALIDATE CONSTRAINT %I;',
    tc.constraint_name,
    tc.table_name,
    tc.constraint_name
  ) AS x
  FROM information_schema.table_constraints AS tc
  JOIN information_schema.tables t ON t.table_name = tc.table_name and t.table_type = 'BASE TABLE'
  JOIN information_schema.key_column_usage AS kcu ON tc.constraint_name = kcu.constraint_name
  JOIN information_schema.constraint_column_usage AS ccu ON ccu.constraint_name = tc.constraint_name
  WHERE  constraint_type = 'FOREIGN KEY'
    AND tc.constraint_schema = 'public'
)
  LOOP
    EXECUTE (r.x);
  END LOOP;
END;
$$;

This will work on all constraints safely.,

SELECT FORMAT(
  'ALTER TABLE %I.%I.%I VALIDATE CONSTRAINT %I;',
  current_database(),
  nsp.nspname,
  cls.relname,
  con.conname
)                                         
FROM pg_constraint AS con
JOIN pg_class AS cls
  ON con.conrelid = cls.oid
JOIN pg_namespace AS nsp
  ON cls.relnamespace = nsp.oid
WHERE convalidated IS FALSE
  -- or delete it for all constraints in all schemas
  AND nsp.nspname = 'mySchema';

You can either save that to a file and Q/A it or execute it all at once if using psql with \gexec.