You can identify a compound noun by its form and function. A compound noun consists of two or more words and it is used as a noun to denote a person, a place, an animal, or an idea.

Whether a compound noun is written as a single word or as separate words, it is always clear that the words that make up the compound are related. For example, most compound nouns contain nouns that are modified by adjectives or other nouns. Take the word basketball and whiteboard as examples. The first compound noun “basketball” contains two nouns: basket and ball. Here, basket simply modifies ball: it tells us the kind of ball being talked about. The second compound noun “whiteboard” is a combination of a noun “board” and an adjective “white” which modifies or describes the board. Thus, the way a compound noun is formed helps us to identify it.

Another thing that helps us to identify a compound noun is that in most cases one of the words specifies the purpose of the other. This is exactly what you find in compound nouns like washing machine, swimming pool, writing pad, payroll, attendance register, teaching aids, instructional materials, ink pad, drinking glass, cooking pot, bathing soap, toilet tissue, kitchen utensils, housemaid, veterinary doctor, surgical knife, and so forth.

There are three types of compound nouns, namely: 1. closed form (gateman), 2. open form (branch manager), and 3. hyphenated form (mother-in-law). However, the simple way to identify each of them is to know that every compound does the job of a single noun: it stands for a person, place, or thing. Consider the above three types of compound nouns one by one. Each of them provides an answer to the question “what?” or “who?”. Who is at the gate? The gateman. Who is heading the branch of the company? The branch manager. Who is taking care of your wife after her childbirth? My mother-in-law. What did you buy for the children? Ice cream. This shows that no matter how complex a compound noun may seem to be, you can easily identify it by bearing in mind that it is a noun in the first place. A noun is a person, place, or thing. So it can always provide answers when we’re asking about “who” or “what”.

For a more detailed audiovisual explanation of compound nouns, their formation, types, usage, and rules, watch the following video lecture:
https://www.youtube.com/watch?v=Wj_SM0O_RO0