Sensing Of Load In Induction Heating
An Induction heater works by inducing Eddie currents in the (bottom of) the pan (Wikipedia article). These currents then cause the pan to heat up.

When a pan is placed on the cooker's coil the electrical properties of that combination change. The electronics that drive the coil can be made such that it detects/measures the voltage across the heater's coil. That information can then be used to detect if a pan is present or not.

Most circuits you can find by Googling for "Induction heating circuit" are from the "hobby scene" and do not have such a detection.

In a commercial product more complex circuits are used, I found this article from NXP showing an example design. In figure 18 of that document there's a "LC resonant Pulse Detection" circuit which I believe is part of this pan/no pan detection.