I am replacing a thermstat and forgot to mark the wires I have two wires and I have air and heat how do I find out what the wires are

I don't know how you can control Heat and Air with 2 wires on a NORMAL/regular Thermostat?
What are the colors? Not that it matters, As it would be make or break.
You can control many things with just 2 conductors, Just not usually done with Central Air.