call them by the names of their fathers. this is the right course in the sight of god. if you do not know their fathers they are then your brothers in religion and your friends. it will not be a sin if you make a mistake unless you do so intentionally for god is forgiving and kind.
ادعوهم لآبائهم هو أقسط عند الله فإن لم تعلموا آباءهم فإخوانكم في الدين ومواليكم وليس عليكم جناح فيما أخطأتم به ولكن ما تعمدت قلوبكم وكان الله غفورا رحيما