Microcontroller - Measure Resistance in Circuit
Most robust would be to measure the voltage across the resistor (differential measurement - perhaps an instrumentation amplifier or such) and the current through the resistor - giving you both unkowns in \$R=\frac{V}{I}\$. The current can be measured with a very small current shunt. 

If you can do with low accuracy, it could be possible to calculate it by using your NMOS in saturation, where it will act as a constant current source. If you then measure the voltage across the resistor you can use the "set" current from the MOSFET and the measured voltage to have a guess at your power. Ofcourse, the error will likely be larger since you are relying on not-so-well-defined parameters of the MOSFET with respect to temperature and process drift.