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.