The libpam-cracklib does it for you. It's a PAM module that is invoked whenever the user changes their password. You can configure it to enforce various password constraints and rules.
Check out section 4.4 for a detailed overview of Cracklib.
Check out section 4.4 for a detailed overview of Cracklib.
Adding a passphrase to your SSH private key is a best practice but not easily enforced without some manual process or command and control for sysadmin workstations.
Hardware tokens containing private keys and used via PKCS#11 are another option for providing some assurance. The token itself can require a PIN.
If you select an easy passphrase for your key, it's similar to selecting a weak password for your bank account when your bank doesn't enforce password constraints. Buyer beware.
An ideal solution would be a two-factor key encryption. That would require the two-factor key to unlock the private key. There might be something out there like that. I haven't looked though.