How to check if power is off to your thermostat

if there is a back-up battery inside the thermostat, remove it. If the display is blank, then there is no power.

Buy or borrow a voltage meter.

Wouldn't the readout disappear? If an older model with no readout, if everything in the area is off, trust the low voltage is too.

Don't flip the breaker. You should have a switch at your furnace. That powers the furnace and thermostat.

Turn thermostat up and if furnace does not come on then there is no power. Can you use a volt meter

No need to cut power to the AC. It runs on its own double breaker. Usually 30 amp 220 V. The thermostat only acts as a switch to turn it on. You don't even have to cut power to the furnace. Once you disconnect the wires from the thermostat power is cut off at the stat. Make sure you label each wire to the letter its connected to. You should have four wires. Connect them to corresponding letters on the new stat back plate. Read the instructions. Insert batteries into the stat snap it on the plate. Program. The batteries just run the display.

Dang, I wrote a response and lost it. Oh, well. I've never lived anywhere with an accurately and completely labeled breaker box unless I did it myself. if the little wires are live they'll only give you a tingle (I think...lol). If no helper, you can check individual outlets by using your hairdryer. Google and see what you can find.

I haven't seen one of those in a long time...if there is no power to the thermostat, the heater or A/C unit will not turn on when the thermostat is dialed up.

Did you get the new stat in yet? If not could you post the process with photos. Would help a lot of members. Some think they need an electrician or a heating guy or just afraid they may mess things up.