We generally assume that +_ 30 degrees from solar south is adequate for most PV production, but for those truly concerned with south facing issues:

solar time = standard time + E +4(Lst - Lloc)

E = the equation in time (in minutes) from equation of time graph
Lst = the standard time longitude line from map of standard time zones
Lloc = the longitude of your location from map of standard time zones

This method means that you can take a single time & date reading at a site & go back and figure solar south. Take the equation, tables & graph with you (and a watch - many cell phones & beepers ought to work too) and figure it on-site. It takes the earth (or the sun, depending on how you want to look at it) 4 minutes to move 1 degree in longitude.

We like compasses, they are cheap - but you do have to account for the earth's magnetic variation . In the middle of Nebraska, there is no variation - sites to the west have an easterly variation - sites to the east have a westerly variation. We use 12 degrees to the west of magnetic north on the east end of Long Island & then reverse it all for solar south. This is not to be confused with a magnetic compass deviation - the hull of a supertanker, your SUV engine block or your great-grandfathers' civil war belt buckle. See Variation Map