"And rend your heart and not your garments. Now return to the Lord your God, for He is gracious and compassionate, slow to anger, abounding in lovingkindness, and relenting of evil. Who knows whether He will not turn and relent, 14And leave a blessing behind Him, even a grain offering and a libation for the Lord your God?" (Joel 2:13-14)
If we ask who knows whether God will turn and relent, the answer is simple. God knows. God knows His own heart and knows what He will do.
We can see that God asks questions even though He already knows the answer to the question. God asking "Who knows whether He will not turn and relent?" does not mean that He didn't know the answer. For example, Jesus asked who the Jews came for in the Garden of Gethsemane when the text clearly states that Jesus knew all things. John 18:3-5 says,
"Judas then, having received the Roman cohort, and officers from the chief priests and the Pharisees, came there with lanterns and torches and weapons. 4Jesus therefore, knowing all the things that were coming upon Him, went forth, and said to them, "Whom do you seek?" 5They answered Him, "Jesus the Nazarene." He *said to them, "I am He." And Judas also who was betraying Him, was standing with them."
Therefore, we can see that God will often ask a question that He knows the answer to. The question is often asked not for God's benefit but for the benefit of the hearers since it brings them to answer the question for themselves, and thereby learn as they are faced with the truth. In this case, the answer is obvious. God knows who will turn and who will relent because God knows all things (1 John 3:20) and no one is hidden from His sight (Heb. 4:13).