All towns are allowed to be off the mark by 15% which is a very big spread. So you need to find comparable homes that match your house and see what they sold for. They have to be true arms length transactions which means no short sales or foreclosures. So say if a three bedroom in Sparta sold for 360,000 the tax assessor is allowed to come in 15 % higher so add another 54,000 to that number and the tax assessor can say your house is worth 414,000.
Not all towns are up to date. Either your house gets reassessed when it sells or every 10 years usually