The total Thomson scatter cross-section is considered in the presence of electron heating and as a function of the ratio of negative ion to electron number densities. For electron densities typical of the lower D region it is shown that the opposing factors of heating and negative ions lead to a local minimum of the cross-section in the experimentally accessible range of 1.5≲Te/Ti≲5. Electron heating is thus considered as a method for detecting negative ions and one early experiment may already have done so.