Yeah, you don't want to use speaker line inputs. Basically you just want to take a known working speaker channel like the front left or right RCA output and connect that to the subwoofer's RCA input. If the sub works and sounds OK, then it is good. If it doesn't make sound, then it is bad.
Then you really need to try to get access to a friend's subwoofer and connect it to your subwoofer out channel and if it works, then your receiver is good (which means your sub is probably bad), and if it doesn't work, then your receiver's sub channel is bad.
With the volt meter, you would need to measure the output on any channels while playing test tones. You can Google it to find descriptions on how to measure this.