I recommend contacting whichever insurance company insurers the vehicle and speak with them. Fault can only be determining by claims adjusters, not the other driver or even police. Give them all information regarding the accident; including pictures. In terms of not being a listed driver on the policy, I would speak with whoever insurers the vehicle and see if they have a "permissive user" clause in their policy. Most policies allow infrequent users (less than 12 times a year, for example) to legally use and operate the vehicle. You can't be a permissive user driver if you live in the same listed residence as the vehicle or use it frequently. For more information on filing a claim and what to expect, see here.