Dorian Iten is a fantastic artist and a devoted educator. His classical atelier-style approach to life and cast drawing places a lot of emphasis on accuracy. To help students learn to draw better from life, Dorian has developed a few tricks for testing and improving one's visual acuity. Check out this brief 3 step process below, and then give it a shot yourself. It's harder than it looks!
And be sure to check out Dorian's site, where he has some really fantastic process and tutorials:
Like 'How to make a Value Study': http://www.dorian-iten.com/value-study/