Five Finger Rule
Pick one random page out of a book to read. Each time the child struggles with a word or skips a word, put down one finger. If you reach the end of the page before you put all five fingers down, then the book is written at a level comfortable for independent reading for that child.
(Reading Levels of Children's Books: How Can You Tell? http://www.hoagiesgifted.org/reading_levels.htm.)