To "find out" a fact means to learn about it through reading or hearing about it from other people.
The object of "find out" comes between "find" and "out" if it's a short word:
How did you find that out?
The object comes after "out" if it's a longer phrase or clause:
She found out that Brad was involved with another woman.
And in some cases, "find out" can be used without an object, like in this example:
I'm not sure. Let me find out.
You can use it this way when it's understood what you are finding out. In this example, you are going to find out the answer to the question.