If user ids are not repeated in each table, then you can use left outer join
:
select u.*,
(c1.id_user is not null) as InCity1,
(c2.id_user is not null) as InCity2
from users u left outer join
city1 c1
on u.id_user = c1.id_user left outer join
city2 c2
on u.id_user = c2.id_user;
You can add a where
clause if you want to see the information for only one user.