Free reading level assessment tests are available at MacmillanReaders.com, K12.com and A2ZHomeSchooling.com. In addition, ThePhonicsPage.org lists three other tests that help determine reading grade levels.
The test at MacmillanReaders.com is an interactive online assessment tool which covers both grammar and vocabulary. Questions appear one at a time, with each accompanied by a picture to aid comprehension.
K12.com lists several downloadable tests based on age. The site provides instructions for administering and scoring the tests.
A2ZHomeSchooling.com offers two simpler tests, both of which consist of lists of words for students to read. ThePhonicsPage.org recommends three other alternatives for assessing reading levels with a description and source link for each test.