In order to understand language, the child must have some background knowledge to use as a reference for interpreting new information. Moreover, if the child is expected to understand something specific, her background knowledge must be relevant to what she is expected to understand.
There are many assessments on the market that measure a child's general knowledge of facts about the world. Usually some estimation is made of what one could reasonably expect children in the first grade to know (e.g. birds build nests in trees, or bicycles have two wheels), and the child is asked to answer these simple "fact" questions (similar to what would be found on the old intelligence tests). However, the most informative assessment is a measure of the child's relevant background knowledge -- specifically, knowledge that is related to the task at hand.
For example, if a child is expected to listen to and understand the story Charlotte's Web, the child should have some background knowledge about farm animals and spiders. Children know a great many things; children raised in the city, for example, know about public transit, taxis, traffic jams, shopping malls, and sky scrapers.
Children raised in other settings know about other things. But any particular child may not know much about a particular topic. It is always worthwhile to assess a child's relevant background knowledge before expecting a child to be able to accomplish a task.