Merriam-Webster defines know as to have information of some kind in your mind, or to have learned (something, as a skill or a language).
It defines find out as to learn by study, observation, or search.
So the point is, know and find out cannot be used interchangeably. Know is not normally used to talk about finding something out. It implies knowledge already acquired. To talk about getting knowledge, we can use find out, get to know, learn, hear.
She is divorced. Where did you find that out?
Why don’t you apply for that overseas teaching position? You’ll be able to see a new place and get to know new people.