Just spitballing, but it's possible that you could take the given password and derive the hashcat invocation that would generate the password. In the strictest case this is equivalent to computing the Kolmogorov complexity of the password, but there's an easier way: have a list of the X most common rules, and test the password against those. Once you have the rule, you can calculate how many possibilities exist in that search space and do a calculation ala GRC Haystack[1]. If it's the case that the list of rules doesn't have one that matches the password, that's probably good enough anyway.

[1] https://www.grc.com/haystack.htm