My first approach would be to casually ask the child to tell me what he/she is reading about at the moment.  Listen to the retelling and accept what you get.  Usually this type of approach enables you to tell if the child is really reading and understanding (comprehending) the text.  If you are not satisfied with that you could ask a few questions to clarify and draw out more detail.  Still not convinced?  Then do it more formally.

Help the child select a suitable book that has not been read before, and where the child is not familiar with its content.  Explain to the child that the task is to read it silently and then tell you about it.  Make sure that the child is focussed and concentrates for the whole reading time.  When the child is ready ask them to tell you all about it.

This is not an examination!  If you quiz the child you will probably make them nervous and they will hate it.  It is a safer strategy to let them tell you what they recall, and perhaps prompt them to expand a little on key points if you must.  As you want the child to improve as a reader you must leave them believing that you are satisfied with their retelling, that they have done a good job of it (provided that they have tried their best).  Remember that your goal is to get them reading for meaning.

If the child cannot read for meaning you must go back to the Listening to Reading activity.