Ateji indication in kokugo headwords

I couldn't seem to find a helpful answer anywhere so I might as well ask here.

In monolingual dictionaries, how is ateji indicated in the headwords? Is there a sort-of standard? E.g. in the sankoku (三省堂国語辞典　第八版）I sometimes see red brackets () around part of the kanji, or all the kanji in a word. Does that indicate ateji, or something else like gikun/jukujikun. In the cases I am talking about, additional information is not given in the gloss.

Thank you in advance