Question on using a multimeter to find a 'c' wire for smart thermostat.

Hey Folks, I had posted this on r/homeimprovement however perhaps this sub is a better place to ask my question:

Hi folks, Im looking at replacing my old 'dumb' thermostat with an smart/wifi thermostat.

To connect a smart thermostat to my system I require a 'common' or 'c' wire.

Upon removing the faceplate from my existing thermostat I see that there are three wires hooked up to the thermostat and one black wire not hooked up to anything.

I understand that I can determine if this black wire is the 'c' wire by using a multimeter to check the voltage on the black wire. I am seeking clarity on how to check this.

Here is a link to an image of my thermostat's wiring: http://imgur.com/8hLmiSW (excuse the poor cut-in on the paint job, I painted knowing that I'd be replacing the thermostat and didnt care about getting it messy)

I have a G, R and W wire wired into the panel. As mentioned I have a black wire not connected to anything.

Can someone confirm that this is what I do to check the voltage on the black wire:

shut off power to heating system remove face plate turn power to heating system back on using multimeter set to detect as close to 24V as possible: -touch one probe to the 'R' wire and the other to the black wire 

If the multimeter shows a voltage close to 24 (18-26V or thereabouts) than Im good to go.

Is the above correct. Particularly, I am supposed to turn the power on then check with multimeter?

Thanks!

edit: Image of my furnace:

http://imgur.com/cFzIcpn

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