Some schools give a screening that is based on academic readiness -- children may be asked to identify colors, letters, numbers, write their name, and count. Other schools give a test that shows developmental maturity through fine motor and social skills -- children are asked to build with blocks, draw pictures, and answer questions about family and friends. Still other schools give tests that are a combination of both academic and developmental readiness.
Whatever kind of screening your son's school uses, the best preparation is to help him feel confident. Kindergarten screening tests are best used to help teachers know where to begin in their work with the children rather than to assist in placement decisions. If your son feels comfortable in separating from you and confident that he will be able to do everything that he needs to do, he will be able to do his best on the screening.