Anyone can tell who commits heresy, simply comparing what they teach with the Magisterial teachings. We are not Church authorities, so we can not sentence anyone, but that should never stop us from exposing heresy for what it is.
At least some do not water down heresy by calling it mere error.
Also, the Catholic Church has never split into groups or factions; people decide to leave the Church by associating with other groups or by adopting beliefs already anathematized.
Yep, you nailed it.