A great reference — onelook.com is a dictionary website. It is most useful for the the FORM of a word. You already know the meaning of swap meet, but is it one word or two words or hyphenated? Put in the entry in each form and see how many dictionaries list it as one or two or hyphenated. For this word, for example, approximately 20-plus dictionaries list it as two words and only a few as one word. You conclude that it is two words.
When the number is even on both sides, default to Merriam-Webster, remembering that the hardback is almost 20 years old. Work with a current subscription of M-W.