Exactly, that's my problem with the quest. You can only find out about their identity after they are dead. Which doesn't help your decision if you are playing a neutral rational character like the one I am playing now.
On my first playthrough, I played a Lolth sworn drow so I found Karlach story appealing and I didn't mind killing who appeared to be a paladin of Tyr.
There should be an obvious hint somewhere in the house or in the surroundings about who they really are.
There is. you can talk to the dead guy lying on the floor near the quest giver who says that they have a pact with a devil. Then if you have Laezal she warns you about getting involved in the affairs of devils. There are also passive checks during your initial convo with Anders that casts doubt on his story. And once you speak to the dead guy you can confront the quest giver about lying about their identities.
