You could have your child take an EPGY Placement Exam in
Secondary Mathematics http://epgy.stanford.edu/applyandregister/upexam.html or take some of the AOPS diagnostic tests http://www.artofproblemsolving.com/School/classlist.php or have your child participate in contests such as AMC 8 to see how he/she compares to mathematically motivated 8th-graders.

"To see what is in front of one's nose needs a constant struggle." - George Orwell