check with the county clerk because there may have been a filing at the time of purchase. but if unsuccessful and if you don't know your basis you should get at least an informal appraisal. you could guess but should the IRS dispute this and contend the basis you used is too high and exceeds the exclusion you're going to either have to pay the additional taxes, penalties and interest assessed or pay a pro to litigate the matter for you which would still cost you with no guarantees as to what the result might be.
