Quoting Jason White (jason@jasonjgw.net):
I can confirm that checks are performed: /etc/pam.d/common-password: password [success=1 default=ignore] pam_unix.so obscure sha512
The obscure option performs the checks listed in the pam_unix(8) manual page.
Eh, you're right. That's the modern implementation. (Obsoletes what I posted a few minutes ago.) People with ongoing users from a long time ago may still be using weak passwords with quaint hashing methods no longer recommended by anyone, e.g., md5 or DES. You can tell by noting the hashing prefix in /etc/shadow. $6 is sha512. Anything lower is a somewhat obsolete hashing method. Not exactly an emergency, though: If someone is able to steal your /etc/shadow, you have much bigger worries than weak password hashes.