Local solar noon: look it up here:
http://www.spot-on-sundials.co.uk/calculator.html
Hang a plumb-bob - (a rock on a string will do) - the shadow points due north-south at that time. It will be more accurate than any magnetic compass and should get you within half a degree.
When setting up my mount, using a compass app in my iPhone and just eyeballing the offset by standing north of my mount and noting some distant mark at a bearing of 169 degrees, typically results in an initial azimuth error less than +/- 2 degrees. Sometimes I'll make the effort to get it below a degree using the dec setting circles.
And yes JB is right - the correct term is magnetic variation - not declination.
Unless theres something steel the size of a car within 15 metres you can forget local influences.