That is great that you are taking an active interest in your children's education. I'm not sure if you live in the United States or elsewhere and what age your children are.
There are a variety of tests available for assessing grade-level and percentile in academic areas. I would suggest beginning by contacting your school's guidance department and requesting that they assess your children's current levels.
There is no one test for assessing grade level and assessing potential is even more complex. In elementary and middle school education, some common tests are:
At the middle school or high school level, schools and programs may also use the ACT, SAT, or SAT II to assess academic knowledge.
If you are in the United States, your children are entitled to a free and appropriate education. Although gifted assessment is separate from academic testing, if your children score well above grade-level you may wish to have them evaluated to see if they qualify as "gifted". Gifted children, like other children with needs that cannot be adequately met by the standard academic program, also qualify for an Individualized Education Program. This may include special services, programs, alternate assignments and/or other accommodations.
If you live outside of the United States, I recommend contacting the department of education to determine how school systems are required to accommodate gifted children and/or what assessments are available for determining your children's academic levels.
I recommend using Bison Educational Systems. This is a site that has questions that are similar to those on the state proficiency tests. Perhaps you can test her using a higher grade and see how she scores on the tests.