Best way to measure an Amp's power capability

Look for nothing, LISTEN/HEAR for what makes the hair on ur neck and arms , STAND UP AMD TPU GET GOOSEBUMPS.

 You will need a minimum of 300 WPC/RMS!
to handle the crescendo’s and song ending power chords!

 Owned so many amps since 82 til the present!

 Every amp under 200wpc have been subpar, and clipped when driving loud music.

 Don’t short yourself.

 Power is needed.

You could search for independent measurements might give you a better idea if the amp meets your needs. This months stereophile looks at a Yamaha integrated the manufacturer specs are pretty close to JA's measured specs. Unless you have the equipment or know someone that's about the only way I know you can get accurate info is with independent testing.

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. 

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

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.

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.

What should one look for?

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

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...

So what is the best way of telling if an amp is high current

Its also a good idea to know what is meant by 'high current'. Many manufacturers specify a current value independently of the output power of the amp, but current can't exist without voltage, and voltage and current together are power. Quite often these 'current' ratings are actually the amount of current present when the power supply is short-circuited and don't represent the output power of the amp at all.


If you are concerned about the amp being able to drive a low impedance load, here are some additional things to keep in mind. First, what is needed is that the amp can act as a 'voltage source' into that load. This simply means that its output voltage will not be affected by the load impedance. Second, you really want to think about what it means to make an amplifier work hard for a living! All amps make more distortion into more difficult loads and that distortion is audible as brightness and harshness. IOW if your goal is high quality audio, you might want to consider working with a speaker is easier to drive, even if the amp you have in mind can be used for welding (perhaps, **especially** if your amp can be used for welding...). Amps that are working hard generally don't sound nearly as good as they could otherwise!

The answer is really that it's complicated.  The issue isn't just a matter of power and current but the speaker impedance and how the amp interacts with it.

Some speakers may have a dip  in the mid bass, causing a dip in amp output.  Well, that may sound really good to your ears! :)

My suggestion is to always listen to an amp/speaker combination, at levels you actually like to listen at.

However, if you want to study this further, take a look at how Stereophile measures amplifier response, especially tube amps and the simulated speaker load.