If you're familiar with the principle of magnetic declination (angle between the geographic and magnetic poles), you can get the current declination value from websites like
http://magnetic-declination.com/ . Not sure just how accurate that site is, but look around and see if there's a better site! Note that declinations vary year-to-year as the magnetic pole wanders a bit.
That should get you approximately pointing south using a compass, but also beware of metallic objects near your compass, perhaps including telescopes (!), which will muck up the accuracy of your pointing if using a magnetic compass (see images here for an example
http://www.trailmeister.com/HelpfulT...usecompass.htm).
Techniques like drift alignment as bartman suggests would probably be necessary to get a more accurate polar alignment, also accounting for the altitude of the pole above the horizon.