Sorry, this is long. I just kind of pooled some information I've picked up over the hatches.
If an egg dies early on, there will be a blood ring around the fetus (I'll attach a picture.) If it dies later in the incubation, it will develop strange brown spots and pooled blood which you will be able to see when candling. In some cases, the ducklings can slice open their own arteries when they move around, which they usually die from. Pooled blood will usually show as a dark spot on the outside of the egg, but this doesn't mean that the chick has died or will die. If eggs ever darken or develop dark spots, make sure to candle them and check if the baby is alive.
On an unrelated note, talk to your eggs in the later weeks of incubation. They can hear as soon as their ears are formed and will respond to familiar noises. It's not drastically important, but I think it's very sweet that they can hatch already attached to your voice. I had one egg where every time I spoke in the last few days of incubation, he would move around in his egg. I could see him move when I candled the egg. As soon as he pipped internally, he would chirp in response to my voice.
When candling the eggs, do it above a soft surface like a thick blanket or a squishy pillow (your bed would also be good, anything that'll break the fall without hurting the embryo and that the egg won't roll off of.) Dropping the eggs is a serious possibility and can be very devastating.