As in any trouble shooting you proceed in a logical manner, in other words using an AC meter using the neutral as a reference logically start at the point you have 120ac, e.g. the fuse and proceed through the circuit at the point you would expect to see the path of the ac supply. If you have the circuit miss-wired then you will see the point of error and likely find another point that shows 120v where it shouldnt be.
If you have the relay in a socket, it is easy to get the terminations mixed up if not carefull.
Make sure you identify the common arm of the relay contacts.
Al.
CNC, Mechatronics Integration and Custom Machine Design
“Logic will get you from A to B. Imagination will take you everywhere.”
Albert E.