The scientific approach (shame on us audiophiles
) would be: Measure the resistence, inductance, capacitance. lower is better.
On a practical note: You'll measure negilible impedence unless your cable is wound into a tight coil. You'll get negligible capacitance unless your cable is hundreds of feet long. Resistence is your only concern. Just use your ears. It is most noticable with woofers in the bass region.
The thicker (and shorter) the speaker wire: The Tighter and more control the amp will have on the speaker, and so it will sound. (snappier,tighter, more detail,more restrictive, lowwer Q) Note : this will not help a speaker play deeper.
The thinner (and longer) the speaker wire: The losser the amplifiers hold on the speaker will be. Allowing the speaker to extend further, play deeper-louder, raises Q) extending deep (rumble) .. Note example : A speakerbox designer can offset a small and constrictive box design by using a thinner wire.
The optimal speaker wire gauge will depend on your taste ,the Speaker design,box design, and amplifier (damping factor)
Thin wire is cheap, give it a lissen on a song with bass, compare from there.
Note: Though true, this doesn't apply in a practical way to low-level signal cables.
-Jeez, With post like these and my budget, I'll never get to call my self an Audiophile again