What is the ruling on saying: "O Allah, if You have answered my prayer, then send down rain upon us" or "O Allah, if You have answered me, then show me a sign" during or after supplication? Is this considered an aggression in supplication or making conditions on Allah?

For a supplicant to ask Allah to show him a sign indicating the acceptance of his supplication is not considered an transgression in supplication (al-i'tida' fi al-du'a'), as a similar supplication was reported from Ibn Umar. There is no harm in this supplication. The most important thing is for the servant to supplicate his Lord and ask Him for his needs. If he is inspired to supplicate, he should know that the answer is with it.