it's not safe to assume any particular wire color scheme was used
Yes and no. While it is possible that an electrician could have used thermostat wiring in some unconventional way, it is more likely to have your house wired properly.
The reason for this is that most (if not all) equipment has primary terminals labeled according to the color of wire that is supposed to be attached to it:
- R, Rh, Rc (power) - Red wire;
- W (heat) - White wire;
- Y (cool) - Yellow wire;
- G (fan) - Green wire.
As a result, it simply does not make any sense for an electrician to invent alternative wiring only to risk accidental short circuits in the process.
how to identify the wires?
The most reliable way, of course, is to look at the terminal block of your heat unit and make sure it is wired according to color scheme above. Pay special attention to the wires connected to "C" terminal - is any of thermostat conductors connected to it? (If yes, then someone has wired "common" to be used at thermostat in unconventional way.)
You don't need to disconnect anything or even use the multimeter on it, just look. But if you don't have an access to the terminal block there is still another way.
First thing to understand, is that the control signals going to heat, AC and fan do not power them. They usually connected to the relays inside said devices. And the other end of the relay coils is connected to common wire coming from the 24V transformer.
So, unless you have some modern devices with SSRs on the inputs, you should see around 24VAC between Red wire and each of the control wires connected to corresponding appliances.
The procedure is simple then: measure voltage between all wire pairs. One wire should give you ~24VAC to each of the other wires, and all those other wires should have 0V between themselves. That one "special" wire would be your Red, and then it is just a matter of figuring out which of the other wires controls the heater.
Note, that if you do not have AC and fan in your house then only one pair of wires will have voltage across, in which case simply use color to connect these two to corresponding terminals. (I suspect thermostat will work properly even if these are reversed by the unscrupulous electrician somewhere).