You can use a current probe to measure the output current and use a voltage probe connect both to the same oscilliscope. That you can do the calculation to get the Resistance. Only if you are measure the dynamics resistance of the power mos.
But normally, we measure it when fully turn on the MOSFET. That means you can just turn on the MOSFET with the operating Vcc and pump in a current source (depending on your rating such as 500mA), and measure the Voltage at the output. The resistance can be calculated. Take care on the max allowable power dissipation of the device when you pump in the currrent on the MOSFET.