We just candled our batch of duck eggs at Day 5, and with a good LED flashlight, it was really clear what was growing, and what wasn't. There was a very definite dark spot and veining on the viable ones, and one that was perfectly clear. We felt very comfortable tossing that one-- it was just so obvious that nothing was growing, and the egg likely just wasn't fertile.
A lot of people here recommend waiting until Day 7, though, which is also a real safe bet if you're not confident in your flashlight, or in your candling abilities!
As far as when to toss, it depends on what's going on with the egg. Most recommend tossing immediately any eggs that are weeping or that smell. Otherwise, the usual recommendation is to keep it unless you're absolutely sure it's no longer viable-- better safe than sorry!