"Find out once and for all" means to determine a matter conclusively and put an end to any doubt or uncertainty. Other synonyms include "settle the matter," "resolve decisively," "reach a definitive conclusion," "uncover the truth," "discover the answer," "ascertain conclusively," "put the issue to rest," and "come to a final determination." Often used in situations of debate or uncertainty, finding out once and for all allows for closure and finality in a matter. Whether it is in a personal, professional, or legal capacity, these synonyms can help convey the importance of finding a definitive answer.