Actually, we do have a good way to determine how someone will perform. The US military has used the ASVAB test for decades, and studies have shown that performance correlates with test results.
Took the pre-asvab in high school back in the 90s, got a perfect score. The recruiters called a lot. Took the full ASVAB at a recruiting station, still got a very high score. Too easy, how can that be a real test? My ACT/SAT scores were good but not perfect.
http://official-asvab.com/validity_res.htm