@Gargron a simple way to do this, could be to check for old popular domains, like caramail.com , aol.com, hotmail.com, yahoo.com, msn.com and so on. at these jurassic times, people never had incentives to build strong passwords, and from now on they have not be warned about how insecure it is to use the same 6 alphanums on all their accounts.

so for some domains, a warn is always welcome :D