With most, you can look at the copyright page. Toward the bottom, you’ll sometimes see “RL” and a number. That’s the reading level.

As others have said, some books print the reading level on the spine, back cover, or on one of the first pages with the copyright info, etc. Most of these are printed specifically by companies that work with schools for the purpose of helping kids get the right reading level though.
is a site that will give the Lexile level of most books – although there are some that are still not on it yet. They also have a place that shows you how Lexile levels correspond to grade and/or age levels.
may also give reading levels, but I don’t remember in what format (Lexile, grade level, age range, etc.).

Usually not, but most online bookstores have the reading level in children’s and juniors level. Also, if your school has AR (which I personally do not like), they label all the books with reading levels.

If there is one, you can find them on the back cover or the spine of the book or inside the flap on hardcover books. If there is no recommended reading level, look at the age of the main character. Very generally speaking, if it’s a teenager, preteens and up are most suited for the reading level. You can also read the description on the back of the book (if it’s a paperback) or inside the flap (if it’s a hardcover). The simplicity or complexity of the description/summary generally gives you an idea of the writing style of the author, and you can decide for yourself if the general plotline seems to be above or below your reading level.

Some books have a reading level on the title page of the book. Other books have the reading level on the cover of the book (as was already noted). I think that the Avon books have the reading level on the cover. Books are not required to have a reading level on them.