。
Who can say what will happen tomorrow?
Literal
Tomorrow what [subject-が] occur [question-か] who [to-に] know [question-のか].
誰にわかるのか is a rhetorical question — 'who could possibly know?' The に marks the person who might know, and のか adds an emotional, rhetorical tone. 何が起こるか is an embedded question ('what will happen') serving as the object of わかる.