Best way to measure an Amp's power capability

Awesome replies and very helpful. You are right. Its just hard to audition now a days but when I do I plan on listening to an amp at very low volumes FIRST. If the sound has PRAT with dead black background and good control then it's a good powerful amp .. i think...

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