Legal Question in Family Law in California

If it was served by the county Child Support Services, it is probably a case of mistaken identity, e.g., someone with the same name. It could however, be fraud too. In either case, work with them to establish that you never had any contact with the mother. They're usually pretty good about dealing with this kind of thing. They are supposed to be neutral helpers to both custodial and non-custodial parents, so they are not usually adversarial. If they start stonewalling you or being adversarial, though, get a lawyer. They have a lot of power and if they decide to go after you it can be hell if you are not represented.