The Prophet, salla Allahu alaihi wa sallam, said: Whoever says to another Muslim O kafir, one of them has to take it.
Are you not hesitant to give takfeer? This is a sign of Alkhawarij!
Things that nullify Islam are well known but they can be applied at individuals without interrogating them and making it clear that there are no obstacles preventing their Takfeer and that all the conditions of giving takfeer have been fulfilled.
This is to be done only by scholars and not by laymen!