Turn off the power to the furnace. Go to the thermostat and carefully remove the cover. Find where the control wires connect to the thermostat and write down the color of the wire and the letter designation of the terminal it connects to. For example, if there is a red wire and it connects to a terminal on the thermostat designated R, you'd write Red--- R. Once you have mapped all the wires, go to the furnace and you will see the same letter designations on the furnace control board as on the thermostat. You now know what color wire goes to which terminal. But double check there have not been any color changes in the wires at the furnace from what you saw at the thermostat. A new wire splice is always possible but unlikely.