We get this question quite often.
Basically your True South satellite is the one that has the same Longitude as your location or the closest one to your location.
For example, if you live in New York, NY, then your Longitude is 74 W. Taking a look at the satellites list here
http://sadoun.com/Sat/Satellites/Satellite-List.htm you will find that SBS6 is the satellite located at 74 W. So that is your True South satellite.
Then based on that, you should consider the magnetic deviation for that location when looking at the compass to establish True South direction for your motorized dish. The Magnetic deviation for New York is +14. That means the True South direction should be equal to 180 +14 = 194 degrees on the compass.