How do you know a loudspeaker is the best?

Listen to really high end speakers so you know what's possible -- then, try to get as much of that; Coherence, natural detail, resolution, sound staging, air, extension, tight bass, musical, etc. as you can at the price range in which you're shopping.

After years of saving, I finally have accumulated enough to buy some top-rate speakers. For the last year, I have been going from dealer to dealer auditioning speakers. I started off with some B & W 705s. Something was missing. Went to another dealer and listened to the B & W 805s & 802s. Better sound, but I still wasn't satisfied. Went to NYC several times. Heard the Revel, Dynaudio,Vandersteen 5A, and Wilson WATT 7. Still wasn't hearing everything I was anctipating. Then I heard the B & W 800s. And all of a sudden I heard everything I wanted to hear in a speaker. (By the way I kept bringing the same CD's to the auditions).
I share this with you not to sell you on the B & W s; but to inform you of the process I went through to find speakers that worked for me.
As some of the other correspondents suggested, keep listening until you find the speakers that work for you.

Your question suggests that you have no basis for evaluating the quality of a speaker. The only thing I can assure you of you don't measure it by its cost, size or type. Design and quality of parts/built are very important but they don't make a speaker "great" either.

If I were to assume that you already had the best electronis and sources, then I would say the speaker that is "best" is the one that sounds best to you in your room. That is YOUR great speaker. But if you don't have the best room, or the best components, that same speaker may produce sounds that lead you to believe it is a POS or some such negative evaluation, but another speaker may work just fine with the same stuff (this indicates a probable mismatch between the speaker and you room and/or electronics.

It's really about your room, your expectations, your finances.

I second Newbee and other's comments about the context of the speaker use. The combination of source, amp and cabling make a huge difference on how a speaker sounds. A balance of budget and a blend of attributes will create a sound that is unique in your room. Making those decisions and discoveries is part of the fun of this hobby for me. I think that experience is also important as you said in your question. I evaluate sound very differently with each major upgrade in equipment.

Trust YOUR ears... grasshopper. As mentioned above, listen to acoustic, not electric, musical instruments and then try to find speakers that closely reproduce those sounds. Drums, cymbals, guitar, piano, violins, etc, are key, somewhat depending on what kind of music you listen to. Horns, of course, are also important, but like vocals, can suffer more from recording imperfections, than actual limitations in the equipment. Listen...it's all good!!!
Sonny