sensor is on left hand side so take side cover off
as for tests you cant be 100{e5d1b7155a01ef1f3b9c9968eaba33524ee81600d00d4be2b4d93ac2e58cec2d} till you change it as it a switch that opens and closes as the water fills. if it makes 1 or 2 every now and then then its still faulty but multi meter wont show it
I would check wiring to it first as had one eaten be mice last year , and take plug off and put it on in case of bad joints
then try it
if all fails its a change it and see what happens
Dave