First off, there is a way you  can tell if the egg is developing, though it's not 100%. Before you set the egg, the yolk is usually up near the air cell, or at least towards the center of the egg. This, when candling, would look like a darker part of the egg in a darker shelled egg (haven't done marans, so I don't know if it would work for them). Anyway, before you can see veins, if the egg is developing, the bottom of the egg will be dark, and the top of the egg will be clear. Usually in a few days you will see veins. 
Update on my eggs:
Silkies, due date 1-23: out of 20 eggs I have 6 white, 3 cuckoo, and 1 buff egg developing. This is good, since I didn't think any buffs were developing!!
Ameracaunas, due date 1-25: Out of 6 eggs, one looks like it has a blood ring, the other eggs are hard to candle, but they are not clear for sure!! 
 
Golden Lakenvelders, due date 1-27: Out of 5 eggs that survived shipping, I have 5 eggs with gorgeous veining!! The other eggs were scrambled, and 3 of those got tossed last night for weeping in my bator 
Hope they all hatch for me, and everyone else's hatch for them!!