Locate one or more math sites that are appropriate for your child's education level. Explore these yourself; try any demos or tests to see whether they make sense to you and would be a good fit for your child.
Let your child explore the sites you have decided to use for testing purposes. Make sure she is comfortable with using the computer to perform math exercises and has time to familiarize herself with some games and skill-building exercises.
Check with your child's teacher or consult the home-schooling curriculum to choose appropriate assessments. Base your choice on the math topics and achievement levels your child is currently working on.
Test your child on a day when he is well rested and is not ill or stressed, and choose a time of day when he is naturally alert. Give your child a healthy snack 15 minutes before testing.