I managed to install some english dictionaries as shown on https://github.com/TeXworks/texworks/wiki/SpellingDictionaries#mac-os-x. Then I was able to select Edit->Spelling->English-UnitedStates(en_US). At that point, all misspelled words were underlined in red.
However, I would like to ask, once you finish writing your document, is there a way to search for any red-undefined misspelled word?
The reason I am asking is to help my students, who will be writing projects in Texworks during the fall semester. When they are done, how can they check the spelling in their full document?