Purchase or find a free reading comprehension test that is appropriate for the student's grade level. For instance, the Gates-MacGinitie Reading Test, which is available for purchase online, has four different levels, from beginning reading to adult reading levels. The Reading Aptitude Assessment Test, which is free for both school and home use, is intended to test reading levels between grades two and 10.
Prepare students for the test by making sure they understand directions and how to answer the questions. Refer to sample questions or demonstrations provided with the test, or create some sample questions of your own.
Administer the test when the test-taker is well-fed and well-rested, to make sure it's an accurate assessment.
Score the test. You may need to send in the answer sheet to have it graded. If you're using an online test, the test results will likely be provided instantaneously after the test is over.