The usual method: heat spot observation through the microscope on an opened package chip, preferably with a heat camera. While looking through the microscope, you slowly increase the current (or the voltage) until you can see a spot - or several of them - which somehow change their appearance: mostly colour, or a slight bulge perhaps.
This is a destructive method, of course - apart from observation through a heat camera. Instead of the latter, you could also apply a thin spray-on film which changes its colour with temperature.