Books written for children and adolescents are leveled, which means that they have a reading level associated with them. Sometimes this reading level is called a Lexile score. (A Lexile score of 8, for example, means that the book is on an 8th-grade level.)
If you can easily read a book and understand it, then you are probably reading a book on or below your reading level. If you have some difficulty reading or understanding a book (either because the vocabulary is difficult for you to understand, or the plot is too complicated for you) then you are probably reading a book above your level. You can roughly identify your own reading level, then, by identifying the lexile score of a book that is difficult for you and one that is easier; your reading level is somewhere between those two books.
In order to find the Lexile score for a book, you can often check the library label on the book or look it up at lexile.com.
You can find a simple word identification chart at homeschooling.gomilpitas.com (see Reference section). This chart contains a series of words that increase in difficulty of reading level. Begin at the top left corner of the chart and read all of the words you can, working from left to right. When you get to a point where you misread or cannot read five words in a row, or you become frustrated with the words, stop.
Add up the number of words you were able to read; your score determines the grade level on which you are comfortable reading. For example, if you score a 65, then you read on a sixth-grade-plus-five-months level. This scale goes from kindergarten to college level, so it is a fairly comprehensive assessment for reading level.
You can also take reading tests online if you want to improve your reading comprehension skills. You can find reading tests for your state that will help you practice for state reading assessments; simply lisit your state's department of education web site to find links. These practice tests will help you improve your reading skills and prepare for upcoming tests you will have to take in school (if you are still in school).
You can also find reading practice tests at college and university websites. The New England Institute of Technology has a few practice tests, for example. You may also find reading tests at curriculum company web sites, such as pearsonlongman.com.