question #3 Sahih Muslim 1065e, Book 12, Hadith 198 : What criteria were used to determine which group was closer to the truth in early Islamic conflicts?

Abu Sa’id al-Khudri reported from the Messenger of Allah (ﷺ) that a group (Khawarij) would emerge from the different parties (the party of Hadrat ‘Ali and the party of Amir Mu’awiya), the group nearer the truth between the two would kill them.