How to test the reading level?

I have a question after reading the post downstairs, about five years old have 5th grade reading level.
And there are some other posts similar, e.g. first grader has 3rd grade level, etc...
My question is how to know which level a kid is? Is there a standard test kind of thing?

I know some books has the level on it, but those are varies from series to series.

Does anybody know?

Sorry cannot input chinese.