Actually, you can draw blood and see if you have appropriate antibody levels for a given disease. We have a few families in our practice who prefer to follow this path instead of subjecting their children to additional vaccines, and so far the schools have accepted the results as proof of immunity.
However, this is an expensive process, especially if you are drawing blood for multiple diseases. And if the antibody levels are not high enough to prove immunity, the child still needs the vaccine, and thus will receive two needle pokes instead of just one. Finally, the goal of booster doses is to produce more memory B and T cells to make the immunity last longer. Even if your child is immune now, that doesn’t guarantee that he or she will be immune five years from now. However, if your child receives a booster dose, he or she is more likely to have long lasting immunity.