I wonder, if there is a Japanese-English dictionary (incl. vice versa), if the word is formal or on the contrary informal. For example: if it is 口語的｛こうごてき｝、堅い｛かたい｝ etc. Hope there is such a dictionary...as I use imiwa, denshi jisho and daijirin dictionaries, but none of them has this info for the words. Nor weblio internet dictionary tells anything about this.
I am not aware of a dictionary that explicitly indicates this. As a general rule to which I can think of no exceptions off the top of my head (some may exist, but none come to me at the moment), words written in kanji are going to be more likely to be used in written Japanese, while words that are all katakana or all hiragana are more likely to be used primarily in spoken Japanese.
Keep in mind that written vs spoken registers are entirely different from informal (plain/辞書形) and humble (謙譲語) or honorific (敬語) forms. The である form of the copula for example, is informal, but only used the written register... unless you're a samurai, in which case I'm sure you meant でござる or similar.