I usually candle in a dark room with a cheap CSI'ish LED flashlight. It's about 3 inches long, and I bought it at WM, I think. Around day 10, I look for veins and vessels--sometimes they're really subtle. As they approach maturity, the egg will be filled with a solid darkness, the air sac will enlarge, and you may see movement.
In determining whether or not an egg is rotten, I candle them, looking for a floating bubble. (For example, if I suspect that an egg is rotten, I'll candle. If I see an obvious bubble in dark fluid--not an embryo--shifts as I gently rock and candle the egg, then I assume it's rotten.) The easiest way to pick out a rotten egg is if it has sticky, yellow-brown seeping in spots.
Also, if you see a "blood line" around the inside of the egg, you have a dead embryo. It'll be a solid line of red all the way around the circumference of the egg. It's obviously not veins or vessels--it's in a complete, predictable line around the inside of the egg, not random like a vein.
Hope this helps!
Chanda