As a first grade teacher, one of my primary goals is to have every child walk out of first grade at or above grade level. I assess every child the first week of school using several assessment tools. My primary tool is to have each child read to me that first week and take a Running Record to assess his/her reading level. In addition, I also assess sight words and whether they know their letters and sounds.
As the year progresses, I do Running Records on a monthly basis with each child. In addition, I often will do a running record on a specific child during Guided Reading time - I want to make sure that every child is making the progress that they need to be making.
I am fortunate that my principal purchased a classroom license for Reading A-Z. I print out the books for my children and put them together and send them home nightly, with a little note that says, "Read to three people and return this book." I, of course, have read the book with them beforehand. When the book comes back to school, a parent volunteer will reread the story with the child.
Finally, for the children who are having difficulties making the progress that I feel they should be making, I tutor them - either during a recess or before/after school.