Once your child starts reading you should remain silent except perhaps for some occasional words of encouragement. Under no circumstances should you read any portion of the book out loud or otherwise assist or prompt your child. While your child is reading, keep track of the approximate percentage of words that he or she is getting right. It is OK if your child occasionally needs two or three tries to get the pronunciation right, but not OK if your child is guessing words that don't appear in the book, even if those words have a similar meaning. Your child should be able to tell you in his or her own words what the text was about.
Capable readers are proud of their ability and they usually enjoy showing it off to adults.
If you observe your child trying to guess which words might fit into the sentence rather than simply saying each word as written, then there is almost certainly a reading problem. If your child tries to construct the story by looking at pictures rather than by reading the text, then there is almost certainly a reading problem. If your child cannot develop a reasonable pace or stumbles frequently or otherwise seems unsure, or seems to get the words right but mispronounces them, then your child has probably not had enough reinforcement of basic reading skills and practice in independent reading. If your child can read the words but cannot tell you what the story is about, then your child is probably working too hard on identifying the words at the expense of comprehension. This is a sign of weak word recognition skills, resulting from too little practice and reinforcement.