Simplified Open Circuit detection
One way is to drive a small current through the heating element and measure the voltage at the drain of the MOSFET.

You could use your remaining GPIO. You haven't specified what voltage VDD is so you would probably require a couple of resistors to match the input of the microcontroller.

This requires that VDD be active in order to check the heating element. I suspect in your question you really meant "Sense that the resistive element is present before you activate it" not before you turn Vdd on.

The circuit to drive a small current through the element could just be a resistor that has a value that is much larger than the resistance of the element. Maybe many kilohm. It would be placed in parallel with the MOSFET. Since you will need a couple of resistors to shift the level down to the micro they will also provide the sensing current.

This resistor network would draw a small current all the time Vdd is present but it would cause negligible heating.

If you had another IO pin you could use it to enable the current used to sense the heating element presence to avoid the power drain.