A scientist could measure the boiling point of a piece of metal the same way he or she would measure the boiling point of any other substance.  You would subject the piece of metal to a measured amount of heat in a controlled fashion.  First, the metal would melt from a solid to a liquid (the melting point).  Then, the liquid metal would start to bubble and produce vapor at the boiling point.  For most chemicals this would not be too difficult a study to perform.  You would only require a heat source like a hotplate and thermometer.  The difficulty in doing this with a metal is that most metals tend to have extremely high melting points.  For example, nickel has a boiling point of almost 3,000 degrees C.  So the difficulty would arise here in obtaining a heat source that could reach that temperature and also obtaining a thermometer that can operate at that temperature.