How do you tell whether a word is a synonym or an antonym?

An example: Words: big and large Their meanings are similar. They can be used interchangeably most of the time And then take Words big and small. Can these be used in the same context interchnageably? No. Because they have very different, opposite meanings. Words that are of opposite meanings to one another are antonyms. Words that are of similar, very close meanings are synonyms.