You can look on the back of most children's paperbacks... it will say R.L 3.4 or something, meaning the fourth month of third grade.
You can also check
www.lexile.com and search for books that your child is reading... the grade level equivalents are here:
http://www.lexile.com/DesktopDefault...=6&tabid=18#18
Do take it all with a big grain of salt... often motivation plays a huge part in what books a child can read, and the systems for ranking books are suspect at best, and "Grade three level" is very different from one dictrict to another...
dar