As Dave said, flip the switch to LFE - in that setting the sub's internal xover is disabled and you will use the xover in the receiver.
I think what you are getting at with respect to measuring frequency response is a Real Time Analyzer. There is a software version I think called TrueRTA that can be downloaded for free. I've never used it so I can't comment on it.
If you have lots of time and really want to do it, you can use the SPL meter too. You'd just have to get a disc with test tones at various frequencies and write down the SPL at each frequency to make your own plot of the frequency response.