By 4-5 days of incubation, veining should be visible through the shell of a light-shelled egg, if you have a bright enough light. I use a Mag-lite. Handle gently and make it quick - while I have personally seen broody hens get up and wander off for a bite, a drink and a stretch for up to 15 minutes at a go (and the eggs survive this neglect), you don't want to let them cool off too much.
At the time of this writing, I have successfully hatched a single duck egg in this incubator (his name is Sherman and he has gone back to the farm to live). I now have 14 chicken eggs in it, with a hatching date of July 30.