It is possible to check if head lice nits are alive by their color. However, without the aid of a magnifying glass or microscope, it is very difficult to tell if they are dead.
Nits are eggs laid by head lice. These eggs attach firmly to the hair shaft close to the scalp. It is actually easier to see nits when a child has lice than the tiny adults crawling on the scalp. Living nits look yellow, tan or brown. It is easy to see the clear or white shell that the nits leave behind once they are born. These shells are probably the most visible part of the infestation because they stay stuck on a child's hair as it grows. Nit shells can mimic dandruff, but they do not shake off from a child's hair like dandruff flakes.
To prevent infestation, it is a good rule of thumb to keep treating the child until no more nits are visible, even if they are simply the shells that are left behind. Many schools have "no nits" policies that keep children at home to prevent other children from getting lice. Children with lice are highly contagious and should be treated as soon as the condition is detected.