What is the ruling on supplicating with: “O Allah, show me a sign that indicates whether my marriage to this person will be good or bad,” for someone who has not yet proposed to me?

Supplicating is permissible, but as long as the suitor has not proposed, you should focus on what benefits you and supplicate to Allah for the betterment of this world and the Hereafter, and for an ease in finding a righteous husband. If he or another proposes, then perform the Istikhara prayer, consult those with experience, and rely on Allah. Avoid preoccupation with thinking about a specific person who has not proposed to you, as it corrupts the heart and distances one from Allah.