The most popular readability tests are:
- The Flesch formulas.
- The Dale–Chall formula.
- The Gunning fog formula.
- Fry readability graph.
- McLaughlin's SMOG formula.
- The FORCAST formula.
- Fountas & Pinnell Level Gradient
Note that publishers usually declare a reading level of the books recommended (from each of their websites and on the copyright page).
Note that authors/writers often determine the level since they are writing to a child audience. Writers Relief is a site that helps writers to determine the readability of their texts.
Finally, I point you to the Fountas & Pinnell Level Gradient system. To verify the readability of the publishers and formula, you could use this: http://www.fountasandpinnell.com/textlevelgradient/
That would give one a 3 point verification of the reading level: (1) publisher, (2) readability formula, (3) F&P Grade Level Gradient. The best verifiers?...the children and their parents through observation of the reading experience.
Remember to also use the Dolch Sight Word lists for grade levels http://www.sightwords.com/sight-words/dolch/