Best way to measure an Amp's power capability

What should one look for?

Wrong question. What should one listen for.
But you kids are having so much fun. Don't mind me. 

Measurements generally give very few useful insights about how something sounds. Unless there’s some specific requirement that most amps can’t handle, I pay little to no attention to measurements because it’s too easy to set the measurements up to look like the result you want. Listen first, then if something really catches your attention, you can spend some time investigating it’s circuitry for answers.

A watt is a watt. Voltage across the resistence of the load -current drawn, multiplied by the voltage. What differs is how the power is measured. RMS (root mean square) is .707 of the maximum power. An amp output into a nominal load of 8 ohms at 1kHz at RMS continuous is a standard measurement. Ceteras paribas, amps should be compared using a common measurement.

You're on the right track, You're looking and learning and listening.

Listen, to the BS, listen with your ears and learn what you want to do. 
THEN put down your hard earned cash.

No need to get mad or unhappy, after the fact. Plan it out a bit.

Room size, partner issues, you know that stuff, all counts.

I have a dog that knows when I'm around other than "Mcintosh" stereo gear. Gets kinda uppity, when I come home! (so material, what are these dogs, coming too?  :-)

Regards

I'm certainly not an expert on this subject. I do have some difficult speakers, Revel Salon 2's (85 db). I am driving them with McIntosh 601 monoblocks. I am using the 4 ohm taps to better match the impedance of the speakers. I can drive these speakers at loud listening levels for long periods of time and my amps will only be warm to the touch. My dealer told me these would be a good choice for the Salons because of the autoformers. Not saying there aren't many other good amps out there that would do just as good. However the point others are making in this discussion are correct. Don't just play the numbers, you need to match the components.