One method of testing an item is to study the changes in proportions of children it correctly at different ages. Unless older children are more successful than younger ones in the item, the item is unsatisfactory in a test based on the concept of mental growth.
A second method of testing an item is to find out whether the results for it correspond to the results on the test as a whole. This can be done by correlating success and failure on the item with the score made on the remaining items. If all items measure something in common, then every single item ought to contribute a score that correlates with the total score.