How to add temperature sensor to Mustool soldering iron?
DON'T do this!!!! You are handling live voltage that can be deadly!

A possible way to do it is to:


Add a switch (relay, MOSFET, TRIAC) that breaks the circuit, power to the heating element.
When the circuit is powerless then it's possible to measure the resistance in the heating element.
The meassured value is then converted in a A/D converter to a digital value.
Programmatically you can now make a table that translate measured resistance to temperature.