It's sometimes difficult to tell. The shells are thicker than chicken or duck eggs. They're white, but goslings are larger too so their bodies block more light. The nice thing though is it really doesn't matter.
Because they're thicker they're less likely to have cracks in them. That means that non starters and quitter both can stay in the incubator in until hatching is complete with very little chance of exploding because the interior is sterile. If an egg is decaying it produces an odor that is easily recognized when you open the incubator, which you should be doing at least fourtimes a day with goose eggs. If you're sure the egg stopped developing you should throw it out. But if there's reason to doubt, continue incubating the egg.
That far along though a live egg should be warm to the touch even after it's removed from the incubator. If you're doing daily misting, when you take the eggs out, let them sit for two minutes without misting to cool. Use an IR thermometer to check temperatures if you have one, or just use your skin to see if any are noticeable cooler. If you do this you don't have to candle eggs that clearly have developing embryos (the ones that stay hot). Mist, wait another 10 minutes and return the warmer eggs to the incubator. Then candle the eggs that were iffy. If there aren't any cracks and the incubator isn't crowded I usually just mark the cooler eggs as probably bad and put them back in. Other factors that come into play in determining whether to continue incubating isif it was shipped, how old it was when incubation started, how much it cost, and any problems I know about with the egg (package was damaged, incubator got too cool, possible genetic concerns, whatever might be retarding development).
If they haven't hatched by 3 days after the others have you can candle again and open them up to see what went wrong. Sometimes you get late developers that will hatch, but it's rare and the hatchling is usually weaker than the others. If you do crack them open it's not always easy to tell what went wrong, and it's kind of depressing too, so I'd recommend just throwing them out.
