How can I be certain of the prayer's answer, even though I know it might be saved for the Day of Judgment?

Allah answers a Muslim's supplication in one of three ways: by expediting the request, by storing up an equivalent amount of good for them, or by averting an equivalent amount of evil from them. This is what is meant by having certainty in the answer when supplicating. If what the supplicant requested is visibly granted, then this is evidence that Allah has answered him. Otherwise, he should magnify his hope in the other two outcomes.