Perhaps you could find an English dictionary which identifies formal words. Take an English dictionary and look up a formal word, like the verb "deem", and see if the dictionary identifies it as formal. When I tried this online, the Macmillan online dictionary identified "deem" as a formal word.
I often use formal words in casual settings because I am an old person, and these were the words I learned and used in my youth as part of normal conversation. Now they are considered archaic and out-of-fashion, but I still use them because I like them!