How to evaluate speakers?

Two sugggestions:
Pay particular attention to the piano passages on the music you've chosen. I believe piano tones (decay etc) is very difficult to reproduce accurately.
Listen to on set of speakers for an extended period of time (maybe 4-5 straight hours) to get a feel for the sound and see if listening fatigue sets in. Then do the same with the other pair, you don't necessarily have to listen to identical music, just something similar.
It's so difficult when we're changing out speakers after a couple of songs over and over again.

Use your ears and preferably listen to each speaker pair with the same equipment; yours.

I agree with Rar1 and Mt10425. As Mt wrote, piano is the most difficult to get right and Rar1 is pointing out specific music with characteristics that when reproduced well "jump out at you."

I suggest you use music that you have heard many times, with which you have an emotional/viceral connection. That way, when you have the speakers placed to their optimum (no small task, I use tape on the floor to remember where each was located for the last listening) you can find the pair that does "it" for you.

Best of luck & good listening.

Dave

Here's a good article that I keep bookmarked:

http://www.audioasylum.com/audio/speakers/messages/96830.html

Some great suggestions above. You might also check out Chesky's "The Ultimate Demonstration Disk." It contains a dozen or so well-recorded pieces, each selected to focus on a particular characteristic (presence, depth, transparency, etc.), and each preceded by an introduction advising what specifically to listen for and how the piece "should" sound on a good system. For me, this CD has been not only useful in comparing gear, but educational as well.