A thermostat is a switch with opinions. “Cool 72°” means it closed a circuit. It does not mean the condenser woke up.
Calling
Display on. Mode Cool or Heat — not Off, not a vacation hold. Setpoint far enough from room temperature that it has a reason to start (about 3°). Fan Auto. If the screen is blank, batteries or a dead C-wire are still in play.
Many systems pause a few minutes after you change something. Wait. Then go look.
Running
Indoor: air at a supply vent. If the fan is set to On, ignore that — you forced the blower. Auto + air moving means G is doing something.
Outdoor (AC / heat pump): fan spinning or a clear hum while it is calling. Silent and still with a calling thermostat is the classic capacitor / contactor / disconnect visit. Hum with a locked fan is the same visit, louder.
Why this split matters
If it is not calling, we are in the house at the plastic rectangle. If it is calling and the outdoor is dead, we are at the disconnect and the contactor, not “maybe add refrigerant.” If both run and the air is still room-temp, we are in split / ice / airflow territory.
The Toolbox asks these in that order on purpose.