How to check whether my neighbour is legal owner of my neighbour flat ?

Hi Guruji's, I am currently facing problem with my pent house flat person who is not giving maintenance of 1000 per month since 2 years. On recent meeting held between owners, 2 were supporting him saying that he will not pay maintenance as he will look after appartment when ever needed. Where those 2 people are dominating persons in appartment and all others were dummy or neutral in any meeting or argument. While I was checking encumberence certificate of all flat to check who were owners I surprisingly found the pent house were registered under a Pharma company where it is nowhere related to him. When I contacted him, in a conversation he stated that I am the owner and flat is under his name and he was staying since 20 years in this appartment. How can I verify whether that property actually related to him or is it legally given rented/ authority to occupy for a period of time ? Taking out my name would cause a problem as I am interfering in his personal matter. How can I do that. Currently there is no association in this appartment and most of them were uneducated and egoistic persons.