I would not mess with her during the day. Sneak out at night, with a head lamp with red light. Wear a long sleeved jacket, and pull the sleeve down over your hand so you can reach under her, grab an egg, pull it into the sleeve with your hand. You can then candle the egg, and place it in a basket that is well lined with a warm towel. Mark each egg as you candle them. Or you can wait till she leaves the nest for a break, sneak half a dozen eggs, bring them inside to candle them, then return the viable ones.
But, here's the ?. Why do you feel the need to candle them? The more you interfere, the more likely you are to disrupt her brooding. Simply leave her to her business, and enjoy the chicks that hatch. A broody hen will often kick the dead eggs out of the nest or even eat them. That is the way God designed the broody system to work. If she comes off the nest with her chicks, and leaves eggs behind, you can then candle them, and decide if there are viable chicks that would benefit from being incubated.