From Longman Dictionary of Contemporary Englishseeing is believingseeing is believing (also I’ll believe it when I see it)BELIEVE used to say that you will only believe that something happens or exists when you actually see it → believe
Examples from the Corpus
seeing is believing• If you're still not convinced, seeing is believing.• We are left with the unspecific practical objection, the objection that is as much to say that only seeing is believing.• But seeing is believing, as they say.• When only seeing is believing the unseen reproductive anatomy of the female can not be an article of faith. 