There's a test for neck stiffness. It doesn't always indicate superior tone, but they do tend to lean towards it. Guitars that have it will stay in tune and hold set-ups better for sure though.
Pluck the low E open and let it ring, then bend the G up a step at the 9th fret without plucking it, only the E is still being heard. If the E drops in pitch the neck is floppy, it there is very little to no drop in pitch you have a good candidate for a nice sounding thicker tone.
P.S. keep your thumb in tight to a normal playing position, don't test with it down under the 4th fret etc...