Examine the manufacturer's MOSFET data sheet to determine which leads on the MOSFET are the gate, drain and source terminals. Study the package schematic and locate on that drawing the corresponding location of each of these terminals on the actual MOSFET package.
Turn off the supply voltage that powers the "on" MOSFET and the associated circuitry it is connected to.
Disconnect the wire or PC trace that connects the gate of the "on" MOSFET to the other electronic components.
Connect a jumper wire between the gate and the source of the MOSFET. Because a direct electrical connection between the gate and source of the MOSFET will ensure that the gate and source voltage is the same, the MOSFET will not be able to be turned on -- and will always be off. When the gate to source voltage of a MOSFET is equal to or near zero volts, a MOSFET will exhibit a very high resistance and will conduct very little current.
Connect an ammeter in series with the drain lead of the terminal. Turn the power back on. Note that with power applied, that the ammeter displays a reading of 0 milliamperes indicating that there is no appreciable current flow and the MOSFET is now off.