This is the hard part. Not all kittens make it. Under the best circumstances (a breeder using a healthy male and female, good nutrition, safe conditions and no exposure to the elements or disease), only about 80 percent of kittens make it. Add the fact that we clumsy humans don't understand kitten, and your orphan survival rate is probably going to be between 25 and 50 percent.
Remember-SOME KITTENS WERE NEVER INTENDED TO SURVIVE. They may have a deformity or simply not have the immunity to be able to advance the species.
Sadly, sometimes the first sign you see that there's a problem is when your kitten takes his last breath. I've had that happen twice. What a shock!
Other signs are loss of appetite, dehydration, lethargy, lack of coordination. I'm sure you other experienced fosters can add to this list. When you see something's not right, you need to take the kitten to the vet right then. It doesn't take but an hour for them to go from playing to dying. In cases like that, it's often the heart that can't support the kitten as it grows.
This actually makes me feel alittle better. I have been beating myself up about bonnie's kittens (we got her and kittens from a kill shelter the day after they were born) not making it. But the dad was her brother, she was sick, and the kitten where sick.