To begin with ask for the title deed from the seller and get it verified. This will help you remain assured if the property is in the name of seller and all rights with respect to the property are vested with him. It's important that you check the orginal deed. It's possible that the original deed would have been pledged for loan and a lawyer must be hired to examine any inconsistencies, if any.
An encumbrance would mean that there is a liability on the property. For example, the owner could have pledged his property for a loan from a bank of a financial institution. An Encumbrance Certificate would ensure that there are no attachment of any kind created on the property.
Property tax receipts
Property taxes are essentially taxes paid to the government. You might want to check with the municipal authorities that all dues are cleared by the seller. Of course, you can also ask for the latest tax receipts from the owner itself. Check the details like owners name on the receipt, date of payment etc.
NOC, if there are multiple owners
It is possible that there could be more than one owner of the property. In such cases you might need to see a no objection certificate from other owners.
Remember while buying a property it's best to be cautious and spend a little on a good lawyer. After all, it's an investment that is for a lifetime and is rather an expensive asset.