Once hatchlings have hatched, eggs shells remain scattered in the nest and in some cases also some unhatched eggs. It is rare to have a 100 percent success rate (all eggs hatched) and methods like excavations are used to see what happened in a turtle nest.
First, one has to get to the nest which is not always easy! Even if you know the location, it happens that nest has shifted a little an it can take some time to find it and in some cases it's never found!
Hatchlings that have hatched leave behind an egg shell (shown in above picture except the top right corner where are unhatched eggs).
Unhatched eggs have to be opened (it can be quite smelly!) and if possible a stage of embryo development is established and documented. There are many reasons why eggs didn't hatched- bacteria, contamination by roots of plants or predation (main predator of hatchlings in Cabo Verde is a ghost crab that preys on eggs and also live hatchlings). In later stages of development hatchlings can suffocate (they must breathe air during pipping).
In some cases, live hatchlings are found in a nest, some of them might have some deformities and their survival will be unfortunately unlikely. But the others might be more lucky and hopefully will make it!