Pick a subject. Testing your grade level will be easier if you focus on a specific subject, rather than the entire curriculum for a grade.
Do a free level test, such as the ones available on the Virginia Standard of Living website. Each state and province has its own curriculum, so try to find a level test from the region in which you live.
Purchase a grade level assessment book. Several states, such as New York, have official standardized tests administered to grade school students. You can find test preparation books at bookstores, or on retail websites such as Amazon.
Try taking an SAT. This is the test that all high school students take to enter college. A good SAT score indicates you are at or above the grade 12 level. If you don't want to spend the money or time to take the actual test, you can take a practice one on websites such as Majortests.com.
Check your child's textbooks. If you have children, try looking through their textbooks and see if you can solve the problems at their grade level. Most grade school textbooks have review tests at the end of each chapter, try one to assess your level. You may also wish to order these textbooks directly from websites such as Alibris.