It is only the way, you could get the information is only from your relatives and friend circle of your parents. Other than that no easy to get the information. There is a chance to get some more information, if you could get either your father or mother's PAN Number, if they have, with regard to asserts (buying or selling) from the Income Tax Office. Try to do it.
Mr. T. Kalaiselvan & Mr. Baala subrahmanyam has guided you properly. apart from their advice, If you can trace out detailed entries in Bank accounts, then from payments, you may come to know if any huge payment is made to anybody. which implies that some investment is made. You can trace out from small payments to utility payments (electric, phone, cable, society maintenance, water charges, municipal taxes) the property details. If you know survey No. or CTS No. then you can try from Talathi office or registration office about the deatails of the owner and about the property.