How is testing of variable resistance?

you can check it using avometer .. you can connect it to resistor lines and varie the possition of the resistor and check the changing values ... if it doesn't change the resistor is damaged ... 

another practical mean is that you connect the output line of your resistor with a small check lamb and check the light change while changing the resistor possition ...

in diesel accelerator you need to identify the exact line you shold work on by checking your circuit diagram..