Learn English – How to detect and understand an idiomatic expression

What is an idiom first?

An idiom is a group of words established by usage as having a meaning not deducible from those of the individual words.

Do you see the clue there?

When you see a group of words that actually don't make any sense in the context if read as normal words, suspect that it could be an idiom.

Take any idiom in a sentence and you'll find it odd i.e. not fitting in the sentence as normal words.

Don't go out; it is raining cats and dogs
Don't beat around the bush, come to the point
A small group of employees have control over the entire organization ~ Yeah! Tail wagging the dog!

If you see all these sentences, the part in italics actually does not go with the flow of the sentence if those words are read individually. What do cats and dogs have to do with rains? What has beating the bush to do anything to come to the point? Why a tail would wag the dog; how is it connected with the organization?

But if you study those idioms, you'll find that they not only perfectly fit in the sentence but also add great flair to the sentences.