What determines it? "Will I be able to afford this college without a scholarship/How close to not getting in am I?" If you need help paying for the school, make sure you get the app in ASAP! If you think you're borderline for even getting in, make sure that app is perfect. If both apply, start early, and get it done perfectly and in early.
Also, just check the requirements for scholarships, which are usually posted somewhere on the school's site. If you're close to, at, or above the requirements, you may as well apply!