。
Have you ever seen hail falling?
Literal
Hail [subject-が] fall [nominalizer-の] [object-を] see past-experience [question-ますか].
雹 (ひょう) is 'hail' — distinct from 霰 (あられ, 'graupel, smaller ice pellets') in meteorology. ~たことがある ('have (past) experience of X') is the standard pattern for 'ever did/have done' questions. The nominalizer の turns '雹が降る' ('hail falls') into a noun clause that can be the object of 見る.