Mosfet, resistance between drain and source in circuit
A MOSFET does not have resistance in the same sense that R1 and R2 do. There is no single number which characterizes the behavior of the drain-source path. Instead, the equivalent resistance (drain-source voltage divided by drain-source current) will depend on 3 things: gate-source voltage, drain-source voltage or current (take your pick) and temperature. For data sheet switching use, gate-source voltage is generally held quite high, like 10 volts, while drain-source current is specified at some high level, where "high" depends on the transistor rating. Than the drain-source voltage is measured and the ratio of voltage to current is calculated to produce a resistance figure, referred to as Rds(on).

So it's perfectly possible to measure the effective resistance of the MOSFET in your drawing if you replace the motor with a resistor or other element which produces the same current as the motor would draw. But it is not (reasonably) possible to accurately determine the effective resistance without actually drawing current. Among other things, it's necessary to take into account of whatever heatsink has been attached to the MOSFET.