Knowing for sure
Isn't really possible because if this asteroid passes through what's known as a gravitational keyhole 2029 it could redirect the asteroid on a collision course with Earth in 2035-6. If we wait until 2029 to take any action we probably won't have enough time to prevent an impact. It would be far easier to push Apophis slightly to make sure it misses the small keyhole than to try to push it to miss the entire planet. The B612 Foundation is advocating a fission powered spacecraft to push (or pull) Apophis as early as 2015 (going on memory with these dates).