这样改：As to whether it is good to test students by examination, the only reliable method to measure students' level...
或者：As to whether it is good to test students by examination, which is the only reliable method to measure students' level...
显然这不是一个完整句,而是一个由 As to 引出的短语,有状语的功用,因此其后还应该有句子的主要部分.