What is your setup?
- Browser: Safari
- -> System Preferences -> Language & Region -> Preferred Language -> add (+) -> English (U.S.)
- Applications -> Dictionary -> Preferences -> activate Oxford American Writer's Thesaurus (American English) / New Oxford American Dictionary (American English) and anything else that makes sense like Wikipedia (English), maybe search and install for other dictionaries.
Are you already handling things like that? Then just go and select some text, right click and choose Look Up "..." to avoid DuckDuckGo all the time. Safari should even tell you the worst mistakes, if you select from menu Edit -> Spelling and Grammar -> check spelling while typing. This is working for me on macOS Sierra, at least not too bad. At least I hope so…