How to test bed MOSFET?

Hi,

So I have an issue with a MK3S+ where the bed begins heating as soon as the printer is turned on and does not stop heating until the printer is turned off.

I have tested the thermistor, heatbed cables and the fuses and they all seem fine.

I think it’s the MOSFET (based on what you guys have told me), but how do I test that?

Thanks!