Cry from a Far Planet by Tom Godwin, Science Fiction, Adventure

A smile of friendship is a baring of the teeth. So is a snarl of menace.
It can be fatal to mistake the latter for the former. Harm an alien being only under circumstances of self-defense.
TRUST NO ALIEN BEING UNDER ANY CIRCUMSTANCES.
--From Exploration Ship's Handbook
The problem of separating the friends from the enemies was a major one in the conquest of space, as many a dead spacer could have testified. A tough job when you could see an alien and judge appearances; far tougher when they were only whispers on the wind.