Well... First of all, you gotta know how does a resonating guitar feel like. Go try out some acoustic guitars, preferably good hi end ones like Taylor or Martin. That should give you a good idea of how a guitar resonates. Note that there are some hi end guitars that don't resonate well.
Now go over to the electric. Strum the strings and feel the way the body vibrates. The thing is, if the guitar resonates well, not only will you feel it vibrating steadily but you can hear it. Its like a shimmering acoustic tone that has sustain.
I've tried some guitars with bad resonance, one was a Gibson LP in Swee Lee, it was like a dead log... I strummed it, the body didn't vibrate much (nothing much here since LPs are big with neck thru, so the body wouldn't vibrate much anyway) and there was a pitiful acoustic tone... very weak and it had no character. I would liken it to a very dead acoustic tone that some really low end guitars (Congress, Kapok) have.
Note that this resonance is more obvious if you play with low gain.... if you are going hi gain metal stuff, its really not needed.
Hope that helps!
