I'd try something untimed. Unfortunately, most of the ones I can think of aren't straight math tests. The WJ-III would probably work, but it will be math + reading/writing as would the WIAT. NWEA MAPS might also work but I'm not sure where you'd get her to take it unless your local school can test her for you.
Are you looking to go with a private tester rather than the school? Has she taken any math achievement tests? Unless you are homeschooling, at age 12, I'd expect that her school should have given her some sort of math achievement test by now.
