Authors use a variety of tools to keep their audiences entertained. They use comparisons, emotion, dialogue and humor. Another common strategy used, not only in text, but in daily conversations, is the inclusion of idioms. Idioms are phrases in which the actual meaning cannot be derived from the literal meaning of the words used. For example, a popular idiom is "It's raining cats and dogs." If you are familiar with this particular phrase, you know that it is raining really hard. However, to a person unfamiliar with the English language, it would not make sense. Idioms add color to otherwise unspectacular text and keep the reader on his toes.
Choose a book or short story to read. To make this more challenging, choose something which you have not read.
Search for phrases, sentences or paragraphs that are colorfully describing an object or event. Idioms are commonly used in a descriptive manner.
Check for words or phrases that cannot be taken literally. "You have a chip on your shoulder" is one example. The literal meaning of this phrase is to hold a grudge. You will know you have found an idiom when the actual phrase does not make sense.
Learn common idioms so that you recognize them when you see them. There are dictionaries designed specifically for idioms, and while some idioms can be defined by the context around them, others can only be understood and recognized from memory.