[KyleGoetz]

There are words that have kanji that are typically not written in kanji by careful writers of Japanese. Sashimister has made this point. NagoYankee made a similar point a year or two ago. allhailhata has made the opposite point before, claiming this is a recent trend. They are all three native speakers.

For example, I was corrected by my Japanese professors when I'd write だいたい in kanji （大体） or ありがとう（有り難う） and such. Similarly, do not write 出来る in kanji, but do write 出来事 in kanji!

Sometimes you even get political correctness debates within the Japanese community: some claim writing こども as 子供 is bad, as it implies the child "belongs" to the adult (as in property), and so it should be written 子ども or こども instead. Similar claims have been made about 主人 (husband, literally "master") and 家内 (wife, literally "inside the house"). But those two are more arguments about the words rather than whether to use kanji or not, IIRC.

You just learn as you go based on when you've seen the kanji used in good writing like newspaper articles, books, etc.