How could you calculate the resistance of a circuit?

A variety of techniques can be used. Node-Voltage and Mesh-current (or Loop-current) methods, for example. See related link for examples. If there is a single voltage source in the model, then find the current supplied, and Resistance = Voltage/Current. {R = V/I}

You could simulate/ model the circuit on a computer then apply the power profile and a current value will be calculated. Or if you have the circuit working, Place an amp meter into the circuit and measure the current. V/i = resistance.