Is There Any Easy Way To Check For Leaness? How Do You Go About It?

A PROPER plug read,
Determining whether you have a WOT fuel pressure drop off.
Listening by ear as you manipulate the sticks under power, and feeling for the "softening" of the motor.

Is reading the plugs considered as accurate as an o2 sensor reading on a dyno? I ask because I just built a 540 and on the last dyno run, at 3700-3800 the 02 sensor was at 15.1 which is scary to me, but we read #6 plug, after a fresh instal, and it was a nice tan. should I trust the plug or the o2 sensor?

I would think you should take more plug readings than just #6 if you are concerned about the accuracy of the O2 sensor. #2 and #7 are generally the leanest.