How to find the Survey numbers for the property ?

Hello everybody, This is Christina from Chennai. It is all about a family dispute. My grandfather owned a property in Tuticorin which is salt pan. It is total extent of 62 Acres. My grandfather has 4 sons and 3 Daughters. All got married and settled down with their respective grandsons and granddaughters. Now the problem is, after demise of my grandfather and grandmother, we don't know what is happening in our land and what revenue is coming and we don't know whether they sold the land are not. but now some people in the village is saying that an portion of land has been sold that is 34 acres out of 62 Acres has been conveyed to one person vide Mortgage. Another information is, 62 Acres of salt pan is in the name of my Grandfather and my grandmother. How can they sold the land when the siblings without their presence. And how to find the Survey numbers of the land to apply encumbrance certificate. (People will question at me for like... Admitting mortgage has been done then how can yu say that i dont have S.no. for the property) !! Can ask the Survey number for the property in RTI ? What is the next step ?