The quick and dirty way is just to feed the filter white noise and plot the output signal's spectrum.   Guaranteed to show the actual performance of the filter, and not somebody's idea of how it ought to be working.

If you need a theoretical curve, I'd start with a look at the [bp~] source to see if you can extract some hints about how the filter is implemented.  It may be quite easy to figure out the poles and zeros if the code is clear and/or documented.

Good luck!
Bill Gribble