How can one tell if the heating/AC system in a home is a single stage or multi-stage? My system is 2 zones. (upstairs & downstairs) with separate thermostats for each. But there is only a single gas furnace and a single AC unit. So does that mean it is a single stage system?

If we assume it's not a heat pump, the presence of W1 ,W2 or Y1 and Y2 generally determine if it's two stage.

There would be 3 gas valve coils for 2 zone heating. Pilot, low and high.

For the outdoor unit there will be 3 wires instead of two connected to the low voltage.

Model # may help.

Ignore Carrier Infinity controls and communicating tstats for now. They are special cases.