It does depend on species to a degree, but with almost all fry the only things that you can look for at this stage is feeding habits, including how they compete for food, and their activity level, all of which can be indicators of general health. Later, you can watch for growth rate, while still keeping an eye out for the ones that seem to dominate in competition for food.
Good luck in keeping track of them at the fry stage, however. Best to you, rick
Fry tend to work out the unhealthy ones on their own, as they can't compete for food. As they grow, look for deformities among the runts and euthanize them. Curved spines, balloon bellies, and belly sliders are the biggest issues.