PostgreSQL has no built-in facilities to make sure that you are using strong passwords.
The best you can do is to make sure that all users' passwords are encrypted, and that your
pg_hba file does not allow logins with a plain password. That is, always use MD5 as login method for users.
For client applications connecting from trusted private networks, either real or virtual (VPN), you may use host based access. That is, if you know that the machine on which the application is running is also not used by some non-trusted individuals. For remote access over public networks, it may be a better idea to use SSL client certificates.
To see which users have unencrypted passwords, use the following query: ...