In Chinese language, the character is the unit for writing. As a result, the boundary of words and simple phrases are really blurry. It is difficult for some foreigners to tell them apart when they learn Mandarin Chinese. Chinese teachers often teach you to tell apart in three aspects to judge whether it is word or phrase. Firstly, we can see whether the components of it can be used independently. If you find one of the components is not the morpheme that forms word independently, the whole structure should be the word. For example, 人in 人民 can be used independently while 民 can‘t be used as word. So 人民 is a word.
The second point we can use to tell them apart is the relationship between the two parts. We usually judge by whether the relationship is close or not and whether some elements can be inserted between them. If the relationship is not so close and you can put some constitutions between them, the structure is a phrase, or it should be the word. When you see the structure 白菜 when you study Chinese, you should know the relationship of 白and 菜 is very close and you can‘t just insert some elements like白的菜. Therefore, it is a word. However, it is not close between 白and 马 and you can add some constituents and say like 白的马. Accordingly, 白马 belongs to a phrase.
The last but not least, we can also tell whether a structure is a word or phrase from the aspect of the relationship between the whole meaning of the structure and the meanings of the components of the structure. If you find you can guess the meaning of the whole structure by adding the meanings of the two parts and the structural relationship together when you learn Chinese words, you should know the structure is a phrase but not a word. On the contrary, the structure is a phrase.
Take 白马 and 白菜as examples. From the meanings of 白and 马, we can guess the meaning of 白马 is 白色的马(white horse). But the meaning of 白菜 is not the combined meanings of 白and 菜. It is just the fixed name of a kind of vegetable. Therefore, the former is phrase and the latter word.