Use the thesaurus service provided by words.bighugelabs.com to get synonyms and antonyms to words.
To set up, get an API key and set it in the Workflow Environment Variables.
You can then check for synonyms and antonyms by calling
ant followed by a space and the word you wish to check against.
Alternatively, select a word and press the shortcut you defined to get the results. Pressing ↵ on a word will paste it to the front most app, which is useful when writing a text and want to change a word in place.
If you want to use one of the results to refine your query, press ⌘↵ (synonyms for the word) or ⌥↵ (antonyms for the word).