On thermostat wires, there is always supposed to be 24 volts AC across "C" and "R" - this powers the thermostat.
Finding the "C" wire, if there is one, and you don't have easy access to the furnace end.
"With the furnace powered, read between "R" and "W", and between "R" and "G". Should be 24ish volts. Then read between "R" and the black wire. Bet a nickel it does not read anything... For AC voltage, you need 2 wires. One is hot, and one is neutral or common ("C"). Your old thermostat is just a switch for one wire ("R"). Some smart thermostats need both because they need to power the controls of the stat, they aren't just mechanically switching the hot wire in to a different wire out."
tags: furnace, vac, test, multimeter