The boiling point of a piece of metal can be measured the same way that the boiling point for any chemical solid or liquid is measure. A sample of the material is heated in a temperature controlled system until it begins to bubble as the liquid turns into a gas. Since metals boil at very high temperatures (hundreds or thousands of degrees Centigrade), this does require highly specialized equipment that your average lab would not have. You would have to have a heat source that could safely reach extremely high temperatures and a thermometer system that could handle the same temperatures. In a real world situation, a scientist would simply look up the boiling point of the metal in question from a compendial source.