Leakage from upper flat to may flat

There is a water leakage from upper flat bathroom to my bathroom and passage. Upper flat owner denies this fact sayin that leakage has started during recent painting work undertaken by society that damaged his toilet srain pipe. I approached society and society got the drain pipe repaired and did water proofing. However still some lickage persists and i showed it to one pedilite waterproofing expert who said its not external lickage but its comming from top bathroom. However upper flat owner strsightaway denied it. Society also now says they have done the repair and its not their concern. How do I prove that leakage is from upper flat? Pedilite provides thermal camera imaging cost is Rs. 2500/- however i am not sure if such proof is accaptable for legal action against the upper flat owner. I want to know who is the autorized person to certify that source of lickage is from upper flat? So that i take legal action against the upper flat owner.