You've identified the key dilemma--to get out from civil liability for the check, you will almost certainly have to report that your father stole your check and forged you signature. If the check was created through the criminal action(s) of another, you should not be liable for it. However, if you do not contact the police and press charges, it will be impossible to avoid liability for this check, because regardless of whether he had permission up front or in advance, you will be ratifying his actions after the fact and implicitly accepting responsibility. So you may have a solid defense, but it will involve saying that your father stole the check and forged your name, which will almost certainly (given the amount involved) be a felony.