For thermostats, black is generally the common side of the transformer. White is hot. And it is AC, not DC. HOWEVER, it's not unusual to find things wired wrong. If doing this by trial and error, try the conventions mentioned above.
Here's a link for color conventions for thermostat wires:
If it were me, I'd pull out a voltmeter, as Bink suggests. Put it on an AC volts scale, and check from the white and black wires to the bare one. If wired as I suggested above, the white wire will show about 24 or 26VAC.