Carol Reese, University of Tennessee Extension explains:

You might take note of where the first frosts spare the plants. You could put thermometers in various parts of the garden and note which ones register the coldest temps during the night. Or, you might try putting out identical cups of water in a variety of spots and looking for ice or thickness of ice after a cold night.
 
Of course, usually walls that face south or west are a good bet, especially if they are brick, stone or concrete. Dark colors absorb heat. Tree canopies reflect radiated heat back down to the ground. Bodies of water, even small water gardens will retain heat, and swimming pools do so in a big way.

Caution: Never fall plant marginally hardy plants, since they will not be well established when cold temps arrive. Spring plant and in fall, nice fluffy mulch.